From e003fed52a982da9bc0ae995e7694b803e3f6af5 Mon Sep 17 00:00:00 2001
From: Steven Hugg <hugg@fasterlight.com>
Date: Tue, 3 Mar 2026 19:43:21 +0100
Subject: [PATCH] gb: hacked some gbdk-2020 header files and got hello.c to
 compile at least

---
 presets/gb/gb/crash_handler.sgb | 481 ++++++++++++++++++++++++++++
 presets/gb/gb/crt0.sgb          | 537 ++++++++++++++++++++++++++++++++
 presets/gb/gb/global.sgb        | 508 ++++++++++++++++++++++++++++++
 presets/gb/gb/sfr.sgb           | 111 +++++++
 presets/gb/hello.c              | 262 ++++++++++++++++
 src/worker/platforms.ts         |   9 +-
 6 files changed, 1906 insertions(+), 2 deletions(-)
 create mode 100644 presets/gb/gb/crash_handler.sgb
 create mode 100644 presets/gb/gb/crt0.sgb
 create mode 100644 presets/gb/gb/global.sgb
 create mode 100644 presets/gb/gb/sfr.sgb
 create mode 100644 presets/gb/hello.c

diff --git a/presets/gb/gb/crash_handler.sgb b/presets/gb/gb/crash_handler.sgb
new file mode 100644
index 00000000..ec08a7f8
--- /dev/null
+++ b/presets/gb/gb/crash_handler.sgb
@@ -0,0 +1,481 @@
+; Crash handler support
+; Original code by ISSOtm
+; Adapted by Toxa from gb-starter-kit: https://github.com/ISSOtm/gb-starter-kit
+
+	.include	"global.s"
+
+	.globl	_font_ibm
+
+	SCRN_X = 160		; Width of screen in pixels
+	SCRN_Y = 144		; Height of screen in pixels
+	SCRN_X_B = 20		; Width of screen in bytes
+	SCRN_Y_B = 18		; Height of screen in bytes
+
+	SCRN_VX = 256		; Virtual width of screen in pixels
+	SCRN_VY = 256		; Virtual height of screen in pixels
+	SCRN_VX_B = 32		; Virtual width of screen in bytes
+	SCRN_VY_B = 32		; Virtual height of screen in bytes
+
+
+	.area _CRASH_HEADER(ABS)
+
+	.org	0x00
+	nop
+	nop
+	rst	0x38
+
+	.org	0x38
+	di
+	jp	___HandleCrash
+
+
+	.area _HOME
+	
+___HandleCrash::
+
+	; We will use VRAM as scratch, since we are going to overwrite it for
+	; screen output anyways. The thing is, we need to turn the LCD off
+	; *without* affecting flags... fun task, eh?
+
+	; Note: it's assumed that this was jumped to with IME off.
+	; Don't call this directly, use `rst Crash`.
+
+	ld	(wCrashA), a	; We need to have at least one working register, so...
+	ldh 	a, (.IE)	; We're also going to overwrite this
+	ld 	(wCrashIE), a
+	ldh 	a, (.LCDC)
+	ld 	(wCrashLCDC), a
+	ld 	a, #LCDCF_ON	; LCDCF_ON Make sure the LCD is turned on to avoid waiting infinitely
+	ldh 	(.LCDC), a
+	ld 	a, #IEF_VBLANK	; IEF_VBLANK
+	ld 	(.IE), a
+	ld 	a, #0		; `xor a` would overwrite flags
+	ld 	(.IF), a	; No point in backing up that register, it's always changing
+	halt			; With interrupts disabled, this will exit when `IE & IF != 0`
+	nop			; Handle hardware bug if it becomes true *before* starting to execute the instruction (1-cycle window)
+
+	; We're now in VBlank! So we can now use VRAM as scratch for some cycles
+	ld	a, #0
+	ldh	(.LCDC), a ; Turn off LCD so VRAM can always be safely accessed
+	; Save regs
+	ld 	(vCrashSP), sp
+	ld 	sp, #vCrashSP
+	push	hl
+	push	de
+	push	bc
+	ld	a, (wCrashA)
+	push	af
+
+	; We need to have all the data in bank 0, but we can't guarantee we were there
+	ldh	a, (.VBK)
+	ld	e, a
+	bit	#0, a
+	jr	z, .bank0
+	; Oh noes. We need to copy the data across banks!
+	ld	hl, #vCrashAF
+	ld	c, #(5 * 2)
+.copyAcross:
+	ld 	b, (hl)
+	xor	a
+	ldh 	(.VBK), a
+	ld 	(hl), b
+	inc	l		; inc hl
+	inc	a		; ld a, 1
+	ldh 	(.VBK), a
+	dec	c
+	jr	nz, .copyAcross
+.bank0:
+	xor	a
+	ldh	(.NR52), a	; Kill sound for this screen
+
+	ldh	(.VBK), a
+	ld	a, e
+	ld	(vCrashVBK), a	; copy vCrashVBK across banks
+
+	ld	a, #1
+	ldh	(.VBK), a
+	ld	hl, #vCrashDumpScreen
+	ld	b, #SCRN_Y_B
+.writeAttrRow:
+	xor 	a
+	ld 	c, #(SCRN_X_B + 1)
+	rst	#0x28 		; .MemsetSmall
+	ld 	a, l
+	add 	a, #(SCRN_VX_B - SCRN_X_B - 1)
+	ld 	l, a
+	dec 	b
+	jr 	nz, .writeAttrRow
+	xor 	a
+	ldh 	(.VBK), a
+
+	; Load palettes
+	ld 	a, #0x03
+	ldh 	(.BGP), a
+	ld 	a, #0x80
+	ldh 	(.BCPS), a
+	xor 	a
+	ld 	c, #.BCPD
+	ldh 	(c), a
+	ldh 	(c), a
+	dec 	a ; ld a, $FF
+	ldh 	(c), a
+	ldh 	(c), a
+	ldh 	(c), a
+	ldh 	(c), a
+	ldh 	(c), a
+	ldh 	(c), a
+
+	ld	a, #(SCRN_VY - SCRN_Y)
+	ldh 	(.SCY), a
+	ld	a, #(SCRN_VX - SCRN_X - 4)
+	ldh	(.SCX), a
+
+	call	loadfont
+
+	; Copy the registers to the dump viewers
+	ld	hl, #vDumpHL
+	ld	de, #vCrashHL
+	ld	c, #4
+	rst	#0x30		; .MemcpySmall
+
+	; We're now going to draw the screen, top to bottom
+	ld	hl, #vCrashDumpScreen
+
+	; First 3 lines of text
+	ld	de, #.header
+	ld	b, #3
+.writeHeaderLine:
+	ld	a, #0x20	; " "
+	ld	(hl+), a
+	ld	c, #19
+	rst	#0x30		; .MemcpySmall
+	ld	a, #0x20	; " "
+	ld	(hl+), a
+	ld	a, l
+	add	a, #(SCRN_VX_B - SCRN_X_B - 1)
+	ld	l, a
+	dec	b
+	jr	nz, .writeHeaderLine
+
+	; Blank line
+	ld	a, #0x20	; " "
+	ld	c, #(SCRN_X_B + 1)
+	rst	#0x28		; .MemsetSmall
+
+	; AF and console model
+	ld	l, #<vCrashDumpScreenRow4
+	ld	c, #4
+	rst	#0x30		; .MemcpySmall
+	pop	bc
+	call	.printHexBC
+	ld	c, #8
+	rst	#0x30		; .MemcpySmall
+	ld	a, (__cpu)
+	call	.printHexA
+	ld	a, #0x20	; " "
+	ld	(hl+), a
+	ld	(hl+), a
+	ld	(hl+), a
+
+	; BC and DE
+	ld	l, #<vCrashDumpScreenRow5
+	ld	c, #4
+	rst	#0x30		; .MemcpySmall
+	pop	bc
+	call	.printHexBC
+	ld	c, #6
+	rst	#0x30		; .MemcpySmall
+	pop	bc
+	call	.printHexBC
+	ld	a, #0x20
+	ld	(hl+), a
+	ld	(hl+), a
+	ld	(hl+), a
+
+	; Now, the two memory dumps
+.writeDump:
+	ld	a, l
+	add	a, #(SCRN_VX_B - SCRN_X_B - 1)
+	ld	l, a
+	ld	c, #4
+	rst	#0x30		; .MemcpySmall
+	pop	bc
+	push	bc
+	call	.printHexBC
+	ld	de, #.viewStr
+	ld	c, #7
+	rst	#0x30		; .MemcpySmall
+	pop	de
+	call	.printDump
+	ld	de, #.spStr
+	bit	#7, l
+	jr	z, .writeDump
+
+	ld	de, #.hwRegsStrs
+	ld	l, #<vCrashDumpScreenRow14
+	ld	c, #6
+	rst	#0x30		; .MemcpySmall
+	ld	a, (wCrashLCDC)
+	call	.printHexA
+	ld	c, #4
+	rst	#0x30		; .MemcpySmall
+	ldh	a, (.KEY1)
+	call	.printHexA
+	ld	c, #4
+	rst	#0x30		; .MemcpySmall
+	ld	a, (wCrashIE)
+	call	.printHexA
+	ld	(hl), #0x20	; " "
+
+	ld	l, #<vCrashDumpScreenRow15
+	ld	c, #7
+	rst	#0x30		; .MemcpySmall
+.writeBank:
+	ld	a, #0x20	; " "
+	ld	(hl+), a
+	ld	a, (de)
+	inc	de
+	ld	(hl+), a
+	cp	#0x20		; " "
+	jr	z, .banksDone
+	ld	a, (de)
+	inc	de
+	ld	c, a
+	ld	a, (de)
+	inc	de
+	ld	b, a
+	ld	a, (bc)
+	call	.printHexA
+	jr	.writeBank
+.banksDone:
+
+	; Start displaying
+	ld	a, #(LCDCF_ON | LCDCF_BG9C00 | LCDCF_BGON)
+	ldh	(.LCDC), a
+
+.loop:
+	; The code never lags, and IE is equal to IEF_VBLANK
+	xor a
+	ldh (.IF), a
+	halt
+
+	jr	.loop
+
+.printHexBC:
+	call	.printHexB
+	ld	a, c
+.printHexA:
+	ld	b, a
+.printHexB:
+	ld	a, b
+	and	#0xF0
+	swap	a
+	add	a, #0x30
+	cp	#0x3a
+	jr	c, 1$
+	add	a, #(0x41 - 0x3a)
+1$:	ld	(hl+), a
+	ld	a, b
+	and	#0x0F
+	add	a, #0x30
+	cp	#0x3a
+	jr	c, 2$
+	add	a, #(0x41 - 0x3a)
+2$:	ld	(hl+), a
+	ret
+
+.printDump:
+	ld	b, d
+	ld	c, e
+	call	.printHexBC
+	ld	a, #0x20	; " "
+	ld	(hl+), a
+	ld	(hl+), a
+	ld	a, e
+	sub	#8
+	ld	e, a
+	ld	a, d
+	sbc	#0
+	ld	d, a
+.writeDumpLine:
+	ld	a, l
+	add	a, #(SCRN_VX_B - SCRN_X_B - 1)
+	ld	l, a
+	ld	a, #0x20	; " "
+	ld	(hl+), a
+.writeDumpWord:
+	ld	a, (de)
+	inc	de
+	call	.printHexA
+	ld	a, (de)
+	inc	de
+	call	.printHexA
+	ld	a, #0x20	; " "
+	ld	(hl+), a
+	bit	4, l
+	jr	nz, .writeDumpWord
+	ld	a, l
+	and	#0x7F
+	jr	nz, .writeDumpLine
+	ret
+
+loadfont:
+	xor	a
+	cpl
+	ld	hl, #0x9000
+	ld	c, #16
+	rst	#0x28 		; .MemsetSmall
+	ld	hl, #(0x9000 + ' ' * 16)
+	ld	c, #16
+	rst	#0x28 		; .MemsetSmall
+
+	ld	de, #(_font_ibm + 2 + '0')	; recode table
+	ld	hl, #(0x9000 + '0' * 16)	; destination
+	push 	hl
+	ld	c, #(16 * 3)
+1$:	
+	ld	a, (de)
+	inc	de
+	push 	de
+	
+	swap	a
+	ld	l, a
+	and 	#0x0f
+	ld	h, a
+	ld	a, l
+	and	#0xf0
+	srl	h
+	rra
+	ld	l, a
+	ld	de, #(_font_ibm + 2 + 128)
+	add	hl, de
+	
+	ld	d, h
+	ld	e, l
+	
+	ldhl	sp, #2
+	ld	a, (hl+)
+	ld	h, (hl)
+	ld	l, a
+	
+	ld	b, #8
+2$:
+	ld	a, (de)
+	cpl
+	inc	de
+	ld	(hl+), a
+	ld	(hl+), a
+
+	dec	b
+	jr	nz, 2$
+	
+	ld	d, h
+	ld	a, l
+	ldhl	sp, #2
+	ld	(hl+), a
+	ld	(hl), d
+	
+	pop	de
+
+	dec	c
+	jr	nz, 1$
+	
+	add	sp, #2
+	ret
+
+.header:
+	;   0123456789ABCDEFGHI  19 chars
+	.ascii	"KERNEL PANIC PLEASE"
+	.ascii	"SEND A CLEAR PIC OF"
+	.ascii	"THIS SCREEN TO DEVS"
+	.ascii	" AF:"
+	.ascii	"  MODEL:"
+	.ascii	" BC:"
+	.ascii	"   DE:"
+	.ascii	" HL:"
+.viewStr:
+	.ascii	"  VIEW:"
+.spStr:
+	.ascii	" SP:"
+.hwRegsStrs:
+	.ascii	" LCDC:"
+	.ascii	" K1:"
+	.ascii	" IE:"
+	.ascii	"  BANK:"
+	.ascii	"R"
+	.dw	__current_bank 
+	.ascii	"V" 
+	.dw	vCrashVBK 
+	.ascii	"W"
+	.db	.SVBK, 0xff
+	.ascii	" "
+
+
+	.area _DATA
+
+wCrashA: 
+	.ds	1		; We need at least one working register, and A allows accessing memory
+wCrashIE: 
+	.ds	1
+wCrashLCDC: 
+	.ds	1
+
+
+	.area _CRASH_SCRATCH(ABS)
+
+	.org	0x9C00
+
+	; Put the crash dump screen at the bottom-right of the 9C00 tilemap, since that tends to be unused space
+	.ds	SCRN_VX_B * (SCRN_VY_B - SCRN_Y_B - 2)	; 2 rows reserved as scratch space
+
+	.ds	SCRN_X_B	; Try not to overwrite the window area
+	.ds	2 * 1		; Free stack entries (we spill into the above by 1 entry, though :/)
+	; These are the initial values of the registers
+	; They are popped off the stack when printed, freeing up stack space
+
+vCrashAF: 
+	.ds	2
+vCrashBC: 
+	.ds	2
+vCrashDE: 
+	.ds	2
+vCrashHL: 
+	.ds	2
+vCrashSP: 
+	.ds	2
+
+	.ds	SCRN_X_B
+vHeldKeys: 
+	.ds	1		; Keys held on previous frame
+vUnlockCounter: 
+	.ds 	1		; How many frames until dumps are "unlocked"
+vWhichDump: 
+	.ds	1
+vDumpHL: 
+	.ds	2
+vDumpSP: 
+	.ds	2
+vCrashVBK: 
+	.ds	1
+	.ds	4		; Unused
+
+	.ds	SCRN_VX_B - SCRN_X_B - 1
+vCrashDumpScreen:
+vCrashDumpScreenRow0  = vCrashDumpScreen +  1 * SCRN_VX_B
+vCrashDumpScreenRow1  = vCrashDumpScreen +  2 * SCRN_VX_B
+vCrashDumpScreenRow2  = vCrashDumpScreen +  3 * SCRN_VX_B
+vCrashDumpScreenRow3  = vCrashDumpScreen +  4 * SCRN_VX_B
+vCrashDumpScreenRow4  = vCrashDumpScreen +  5 * SCRN_VX_B
+vCrashDumpScreenRow5  = vCrashDumpScreen +  6 * SCRN_VX_B
+vCrashDumpScreenRow6  = vCrashDumpScreen +  7 * SCRN_VX_B
+vCrashDumpScreenRow7  = vCrashDumpScreen +  8 * SCRN_VX_B
+vCrashDumpScreenRow8  = vCrashDumpScreen +  9 * SCRN_VX_B
+vCrashDumpScreenRow9  = vCrashDumpScreen + 10 * SCRN_VX_B
+vCrashDumpScreenRow10 = vCrashDumpScreen + 11 * SCRN_VX_B
+vCrashDumpScreenRow11 = vCrashDumpScreen + 12 * SCRN_VX_B
+vCrashDumpScreenRow12 = vCrashDumpScreen + 13 * SCRN_VX_B
+vCrashDumpScreenRow13 = vCrashDumpScreen + 14 * SCRN_VX_B
+vCrashDumpScreenRow14 = vCrashDumpScreen + 15 * SCRN_VX_B
+vCrashDumpScreenRow15 = vCrashDumpScreen + 16 * SCRN_VX_B
+vCrashDumpScreenRow16 = vCrashDumpScreen + 17 * SCRN_VX_B
+vCrashDumpScreenRow17 = vCrashDumpScreen + 18 * SCRN_VX_B
diff --git a/presets/gb/gb/crt0.sgb b/presets/gb/gb/crt0.sgb
new file mode 100644
index 00000000..b73d695d
--- /dev/null
+++ b/presets/gb/gb/crt0.sgb
@@ -0,0 +1,537 @@
+        .include        "global.sgb"
+
+        ;; ****************************************
+        ;; Beginning of module
+        ;; BANKED: checked
+        .title  "Runtime"
+        .module Runtime
+        .area   _HEADER (ABS)
+
+        ;; RST vectors
+;       .org    0x00            ; Trap, utilized by crash_handler.h
+
+;       .org    0x08            ; --profile handler utilized by emu_debug.h
+
+;       .org    0x10            ; empty
+
+;       .org    0x18            ; empty
+
+        .org    0x20            ; RST 0x20 == call HL
+.call_hl::
+        JP      (HL)
+
+        .org    0x28            ; zero up to 256 bytes in C pointed by HL
+.MemsetSmall::
+        LD      (HL+),A
+        DEC     C
+        JR      NZ,.MemsetSmall
+        ret
+
+        .org    0x30            ; copy up to 256 bytes in C from DE to HL
+.MemcpySmall::
+        LD      A, (DE)
+        LD      (HL+), A
+        INC     DE
+        DEC     C
+        JR      NZ,.MemcpySmall
+        RET
+
+;       .org    0x38            ; crash handler utilized by crash_handler.h
+
+        ;; Hardware interrupt vectors
+        .org    0x40            ; VBL
+.int_VBL:
+        PUSH    AF
+        PUSH    HL
+        LD      HL,#.int_0x40
+        JP      .int
+
+;       .org    0x48            ; LCD
+
+;       .org    0x50            ; TIM
+
+;       .org    0x58            ; SIO
+
+;       .org    0x60            ; JOY
+
+;       .org    0x70
+        ;; space for drawing.s bit table
+
+        .org    0x80
+.int::
+        PUSH    BC
+        PUSH    DE
+1$:
+        LD      A,(HL+)
+        OR      (HL)
+        JR      Z,.int_tail
+        PUSH    HL
+        LD      A,(HL-)
+        LD      L,(HL)
+        LD      H,A
+        RST     0x20            ; .call_hl
+        POP     HL
+        INC     HL
+        JR      1$
+_wait_int_handler::    
+        ADD     SP,#4
+.int_tail:
+        POP     DE
+        POP     BC
+        POP     HL
+
+        ;; we return at least at the beginning of mode 2
+        WAIT_STAT
+
+        POP     AF
+        RETI
+
+        ;; VBlank default interrupt routine
+__standard_VBL_handler::
+.std_vbl:
+        LD      HL,#.sys_time
+        INC     (HL)
+        JR      NZ,2$
+        INC     HL
+        INC     (HL)
+2$:
+        CALL    .refresh_OAM
+        
+        LD      A, #1
+        LDH     (.vbl_done),A
+        RET
+
+_refresh_OAM::
+        WAIT_STAT
+        LD      A, #>_shadow_OAM
+        JP      .refresh_OAM + (.refresh_OAM_DMA - .start_refresh_OAM)
+
+.clear_WRAM:
+        PUSH    DE
+        XOR     A
+        LD      BC, #l__DATA
+        LD      HL, #s__DATA
+        CALL    .memset_simple
+
+        LD      A, #>_shadow_OAM
+        LDH     (__shadow_OAM_base), A
+        LD      H, A
+        XOR     A
+        LD      L, A
+        LD      C, #(40 << 2)   ; 40 entries 4 bytes each
+        RST     0x28
+        POP     DE
+        RET
+        
+        ;; GameBoy Header
+
+        ;; DO NOT CHANGE...
+        .org    0x100
+.header:
+        JR      .code_start
+
+        ;; Nintendo logo
+        .org    0x104
+        .byte   0xCE,0xED,0x66,0x66
+        .byte   0xCC,0x0D,0x00,0x0B
+        .byte   0x03,0x73,0x00,0x83
+        .byte   0x00,0x0C,0x00,0x0D
+        .byte   0x00,0x08,0x11,0x1F
+        .byte   0x88,0x89,0x00,0x0E
+        .byte   0xDC,0xCC,0x6E,0xE6
+        .byte   0xDD,0xDD,0xD9,0x99
+        .byte   0xBB,0xBB,0x67,0x63
+        .byte   0x6E,0x0E,0xEC,0xCC
+        .byte   0xDD,0xDC,0x99,0x9F
+        .byte   0xBB,0xB9,0x33,0x3E
+
+        ;; Title of the game
+        .org    0x134
+        .asciz  "Title"
+
+        .org    0x144
+        .byte   0,0,0
+
+        ;; Cartridge type is ROM only
+        .org    0x147
+        .byte   0
+
+        ;; ROM size is 32kB
+        .org    0x148
+        .byte   0
+
+        ;; RAM size is 0kB
+        .org    0x149
+        .byte   0
+
+        ;; Maker ID
+        .org    0x14A
+        .byte   0x00,0x00
+
+        ;; Version number
+        .org    0x14C
+        .byte   0x01
+
+        ;; Complement check
+        .org    0x14D
+        .byte   0x00
+
+        ;; Checksum
+        .org    0x14E
+        .byte   0x00,0x00
+
+        ;; ****************************************
+        .org    0x150
+        
+        ;; soft reset: falldown to .code_start
+.reset::
+_reset::
+        LD      A, (__is_GBA)
+        LD      B, A
+        LD      A, (__cpu)
+
+        ;; Initialization code
+.code_start::
+        DI                      ; Disable interrupts
+        LD      D, A            ; Store CPU type in D
+        LD      E, B            ; Store GBA flag in E 
+        ;; Initialize the stack
+        LD      SP, #.STACK
+
+        CALL    .clear_WRAM
+
+;       LD      (.mode),A       ; Clearing (.mode) is performed when clearing RAM
+
+        ;; Store CPU type
+        LD      A, D
+        LD      (__cpu), A
+        CP      #.CGB_TYPE
+        JR      NZ, 1$
+        XOR     A
+        SRL     E
+        RLA
+        LD      (__is_GBA), A
+1$:
+        ;; Turn the screen off
+        CALL    .display_off
+
+        XOR     A
+        ;; Initialize the display
+        LDH     (.SCY),A
+        LDH     (.SCX),A
+        LDH     (.STAT),A
+        LDH     (.WY),A
+        LD      A,#0x07
+        LDH     (.WX),A
+
+        ;; Copy refresh_OAM routine to HRAM
+        LD      DE,#.start_refresh_OAM                          ; source
+        LD      HL,#.refresh_OAM                                ; dest
+        LD      C,#(.end_refresh_OAM - .start_refresh_OAM)      ; size
+        RST     0x30                                            ; call .MemcpySmall
+
+        ;; Clear the OAM by calling refresh_OAM
+        CALL    .refresh_OAM
+
+        ;; Install interrupt routines
+        LD      BC,#.std_vbl
+        CALL    .add_VBL
+
+        ;; Standard color palettes
+        LD      A,#0b11100100   ; Grey 3 = 11 (Black)
+                                ; Grey 2 = 10 (Dark grey)
+                                ; Grey 1 = 01 (Light grey)
+                                ; Grey 0 = 00 (Transparent)
+        LDH     (.BGP),A
+        LDH     (.OBP0),A
+        LD      A,#0b00011011
+        LDH     (.OBP1),A
+
+        ;; Turn the screen on
+        LD      A,#(LCDCF_ON | LCDCF_WIN9C00 | LCDCF_WINOFF | LCDCF_BG8800 | LCDCF_BG9800 | LCDCF_OBJ8 | LCDCF_OBJOFF | LCDCF_BGOFF)
+        LDH     (.LCDC),A
+        XOR     A
+        LDH     (.IF),A
+        LD      A,#.VBL_IFLAG   ; switch on VBlank interrupt only
+        LDH     (.IE),A
+
+        LDH     (__current_bank),A      ; current bank is 1 at startup
+
+        XOR     A
+
+        LD      HL,#.sys_time
+        LD      (HL+),A
+        LD      (HL),A
+
+        LDH     (.NR52),A       ; Turn sound off
+
+        CALL    gsinit
+
+        EI                      ; Enable interrupts
+
+        ;; Call the main function
+        CALL    _main
+_exit:: 
+99$:
+        HALT
+        NOP
+        JR      99$             ; Wait forever
+
+_set_interrupts::
+        DI
+        LDA     HL,2(SP)        ; Skip return address
+        XOR     A
+        LDH     (.IF),A         ; Clear pending interrupts
+        LD      A,(HL)
+        EI                      ; Enable interrupts
+        LDH     (.IE),A         ; interrupts are still disabled here
+        RET
+
+        ;; Copy OAM data to OAM RAM
+.start_refresh_OAM:
+        LDH     A,(__shadow_OAM_base)
+        OR      A
+        RET     Z
+.refresh_OAM_DMA:
+        LDH     (.DMA),A        ; Put A into DMA registers
+        LD      A,#0x28         ; We need to wait 160 ns
+1$:
+        DEC     A
+        JR      NZ,1$
+        RET
+.end_refresh_OAM:
+
+        .org    .MODE_TABLE
+        ;; Jump table for modes
+        RET
+
+        ;; ****************************************
+
+        ;; Ordering of segments for the linker
+        ;; Code that really needs to be in bank 0
+        .area   _HOME
+        ;; Similar to _HOME
+        .area   _BASE
+        ;; Code
+        .area   _CODE
+        ;; #pragma bank 0 workaround
+        .area   _CODE_0
+        ;; Constant data
+        .area   _LIT
+;       ;; since _CODE_1 area base address is pre-defined in the linker from 0x4000, 
+;       ;; that moves initializer code and tables out of bank 0
+;       .area   _CODE_1
+        ;; Constant data, used to init _DATA
+        .area   _INITIALIZER
+        ;; Code, used to init _DATA
+        .area   _GSINIT 
+        .area   _GSFINAL
+        ;; Uninitialised ram data
+        .area   _DATA
+        .area   _BSS
+        ;; Initialised in ram data
+        .area   _INITIALIZED
+        ;; For malloc
+        .area   _HEAP
+
+        .area   _DATA
+.start_crt_globals:
+
+__cpu::
+        .ds     0x01            ; GB type (GB, PGB, CGB)
+__is_GBA::
+        .ds     0x01            ; detect GBA
+.mode::
+        .ds     0x01            ; Current mode
+.sys_time::
+_sys_time::
+        .ds     0x02            ; System time in VBL units
+.int_0x40::
+        .blkw   0x0A            ; 4 interrupt handlers (built-in + user-defined)
+
+.end_crt_globals:
+
+        .area   _HRAM (ABS)
+
+        .org    0xFF90  
+__current_bank::        ; Current bank
+        .ds     0x01
+.vbl_done:
+        .ds     0x01            ; Is VBL interrupt finished?
+__shadow_OAM_base::
+        .ds     0x01
+
+        ;; Runtime library
+        .area   _GSINIT
+gsinit::
+        ;; initialize static storage variables
+        LD      BC, #l__INITIALIZER
+        LD      HL, #s__INITIALIZER
+        LD      DE, #s__INITIALIZED
+        call    .memcpy_simple         
+
+        .area   _GSFINAL
+        ret
+
+        .area   _HOME
+
+        ;; fills memory at HL of length BC with A, clobbers DE
+.memset_simple::
+        LD      E, A
+        LD      A, B
+        OR      C
+        RET     Z
+        LD      (HL), E
+        DEC     BC
+        LD      D, H
+        LD      E, L
+        INC     DE
+
+        ;; copies BC bytes from HL into DE
+.memcpy_simple::
+        LD      A, B
+        OR      C
+        RET     Z
+
+        SRL     B
+        RR      C
+        JR      NC,3$
+        LD      A, (HL+)
+        LD      (DE), A
+        INC     DE
+3$:
+        INC     B
+        INC     C
+        JR      2$
+1$:
+        LD      A, (HL+)
+        LD      (DE), A
+        INC     DE
+        LD      A, (HL+)
+        LD      (DE), A
+        INC     DE
+2$:
+        DEC     C
+        JR      NZ,1$
+        DEC     B
+        JR      NZ,1$   
+4$:
+        RET
+
+        ;; Remove interrupt routine in BC from the VBL interrupt list
+        ;; falldown to .remove_int
+.remove_VBL::
+        LD      HL,#.int_0x40
+
+        ;; Remove interrupt BC from interrupt list HL if it exists
+        ;; Abort if a 0000 is found (end of list)
+.remove_int::
+1$:
+        LD      A,(HL+)
+        LD      E,A
+        LD      D,(HL)
+        INC     HL
+        OR      D
+        RET     Z               ; No interrupt found
+
+        LD      A,E
+        CP      C
+        JR      NZ,1$
+        LD      A,D
+        CP      B
+        JR      NZ,1$
+
+        LD      D,H
+        LD      E,L
+        DEC     DE
+        DEC     DE
+
+        ;; Now do a memcpy from here until the end of the list
+2$:
+        LD      A,(HL+)
+        LD      (DE),A
+        LD      B,A
+        INC     DE
+        LD      A,(HL+)
+        LD      (DE),A
+        INC     DE
+        OR      B
+        RET     Z
+        JR      2$
+
+        ;; Add interrupt routine in BC to the VBL interrupt list
+        ;; falldown to .add_int
+.add_VBL::
+        LD      HL,#.int_0x40
+
+        ;; Add interrupt routine in BC to the interrupt list in HL
+.add_int::
+1$:
+        LD      A,(HL+)
+        OR      (HL)
+        JR      Z,2$
+        INC     HL
+        JR      1$
+2$:
+        LD      A,B
+        LD      (HL-),A
+        LD      (HL),C
+        RET
+
+        ;; Wait for VBL interrupt to be finished
+.wait_vbl_done::
+_wait_vbl_done::
+        ;; Check if the screen is on
+        LDH     A,(.LCDC)
+        AND     #LCDCF_ON
+        RET     Z               ; Return if screen is off
+        XOR     A
+        LDH     (.vbl_done),A   ; Clear any previous sets of vbl_done
+1$:
+        HALT                    ; Wait for any interrupt
+        NOP                     ; HALT sometimes skips the next instruction
+        LDH     A,(.vbl_done)   ; Was it a VBlank interrupt?
+        ;; Warning: we may lose a VBlank interrupt, if it occurs now
+        OR      A
+        JR      Z,1$            ; No: back to sleep!
+        RET
+
+.display_off::
+_display_off::
+        ;; Check if the screen is on
+        LDH     A,(.LCDC)
+        AND     #LCDCF_ON
+        RET     Z               ; Return if screen is off
+1$:                             ; We wait for the *NEXT* VBL 
+        LDH     A,(.LY)
+        CP      #0x92           ; Smaller than or equal to 0x91?
+        JR      NC,1$           ; Loop until smaller than or equal to 0x91
+2$:
+        LDH     A,(.LY)
+        CP      #0x91           ; Bigger than 0x90?
+        JR      C,2$            ; Loop until bigger than 0x90
+
+        LDH     A,(.LCDC)
+        AND     #~LCDCF_ON
+        LDH     (.LCDC),A       ; Turn off screen
+        RET
+
+_remove_VBL::
+        PUSH    BC
+        LDA     HL,4(SP)        ; Skip return address and registers
+        LD      A,(HL+)
+        LD      C,A
+        LD      B,(HL)
+        CALL    .remove_VBL
+        POP     BC
+        RET
+        
+_add_VBL::
+        PUSH    BC
+        LDA     HL, 4(SP)       ; Skip return address and registers
+        LD      A,(HL+)
+        LD      C,A
+        LD      B,(HL)
+        CALL    .add_VBL
+        POP     BC
+        RET
diff --git a/presets/gb/gb/global.sgb b/presets/gb/gb/global.sgb
new file mode 100644
index 00000000..573817cf
--- /dev/null
+++ b/presets/gb/gb/global.sgb
@@ -0,0 +1,508 @@
+        .NEAR_CALLS = 1         ; <near_calls> - tag so that sed can change this
+
+        ;; Changed by astorgb.pl to 1
+        __RGBDS__       = 0
+
+        _VRAM           = 0x8000 ; $8000->$9FFF
+        _VRAM8000       = 0x8000
+        _VRAM8800       = 0x8800
+        _VRAM9000       = 0x9000
+        _SCRN0          = 0x9800 ; $9800->$9BFF
+        _SCRN1          = 0x9C00 ; $9C00->$9FFF
+        _SRAM           = 0xA000 ; $A000->$BFFF
+        _RAM            = 0xC000 ; $C000->$CFFF / $C000->$DFFF
+        _RAMBANK        = 0xD000 ; $D000->$DFFF
+        _OAMRAM         = 0xFE00 ; $FE00->$FE9F
+        _IO             = 0xFF00 ; $FF00->$FF7F,$FFFF
+        _AUD3WAVERAM    = 0xFF30 ; $FF30->$FF3F
+        _HRAM           = 0xFF80 ; $FF80->$FFFE
+
+        ;; MBC Equates
+
+        .MBC1_ROM_PAGE  = 0x2000 ; Address to write to for MBC1 switching
+        .MBC_ROM_PAGE   = 0x2000 ; Default platform MBC rom switching address
+
+        rRAMG           = 0x0000 ; $0000->$1fff
+        rROMB0          = 0x2000 ; $2000->$2fff
+        rROMB1          = 0x3000 ; $3000->$3fff - If more than 256 ROM banks are present.
+        rRAMB           = 0x4000 ; $4000->$5fff - Bit 3 enables rumble (if present)
+
+        ;;  Keypad
+        .UP             = 0x04
+        .DOWN           = 0x08
+        .LEFT           = 0x02
+        .RIGHT          = 0x01
+        .A              = 0x10
+        .B              = 0x20
+        .SELECT         = 0x40
+        .START          = 0x80
+
+        .P14            = 0x10
+        .P15            = 0x20
+
+        ;;  Screen dimensions
+        .MAXCURSPOSX    = 0x13  ; In tiles
+        .MAXCURSPOSY    = 0x11
+
+        .SCREENWIDTH    = 0xA0
+        .SCREENHEIGHT   = 0x90
+        .MINWNDPOSX     = 0x07
+        .MINWNDPOSY     = 0x00
+        .MAXWNDPOSX     = 0xA6
+        .MAXWNDPOSY     = 0x8F
+
+        ;; Hardware registers
+
+        .P1             = 0x00  ; Joystick: 1.1.P15.P14.P13.P12.P11.P10
+        rP1             = 0xFF00
+
+        P1F_5           = 0b00100000 ; P15 out port, set to 0 to get buttons
+        P1F_4           = 0b00010000 ; P14 out port, set to 0 to get dpad
+        P1F_3           = 0b00001000 ; P13 in port
+        P1F_2           = 0b00000100 ; P12 in port
+        P1F_1           = 0b00000010 ; P11 in port
+        P1F_0           = 0b00000001 ; P10 in port
+
+        P1F_GET_DPAD    = 0b00100000
+        P1F_GET_BTN     = 0b00010000
+        P1F_GET_NONE    = 0b00110000
+
+        .SB             = 0x01  ; Serial IO data buffer
+        rSB             = 0xFF01
+
+        .SC             = 0x02  ; Serial IO control register
+        rSC             = 0xFF02
+
+        .DIV            = 0x04  ; Divider register
+        rDIV            = 0xFF04
+
+        .TIMA           = 0x05  ; Timer counter
+        rTIMA           = 0xFF05
+
+        .TMA            = 0x06  ; Timer modulo
+        rTMA            = 0xFF06
+
+        .TAC            = 0x07  ; Timer control
+        rTAC            = 0xFF07
+
+        TACF_START      = 0b00000100
+        TACF_STOP       = 0b00000000
+        TACF_4KHZ       = 0b00000000
+        TACF_16KHZ      = 0b00000011
+        TACF_65KHZ      = 0b00000010
+        TACF_262KHZ     = 0b00000001
+
+        .IF             = 0x0F  ; Interrupt flags: 0.0.0.JST.SIO.TIM.LCD.VBL
+        rIF             = 0xFF0F
+
+        .NR10           = 0x10  ; Sound register
+        rNR10           = 0xFF10
+        rAUD1SWEEP      = 0xFF10
+
+        AUD1SWEEP_UP    = 0b00000000
+        AUD1SWEEP_DOWN  = 0b00001000
+
+        .NR11           = 0x11  ; Sound register
+        rNR11           = 0xFF11
+        rAUD1LEN        = 0xFF11
+
+        .NR12           = 0x12  ; Sound register
+        rNR12           = 0xFF12
+        rAUD1ENV        = 0xFF12
+
+        .NR13           = 0x13  ; Sound register
+        rNR13           = 0xFF13
+        rAUD1LOW        = 0xFF13
+
+        .NR14           = 0x14  ; Sound register
+        rNR14           = 0xFF14
+        rAUD1HIGH       = 0xFF14
+
+        .NR21           = 0x16  ; Sound register
+        rNR21           = 0xFF16
+        rAUD2LEN        = 0xFF16
+
+        .NR22           = 0x17  ; Sound register
+        rNR22           = 0xFF17
+        rAUD2ENV        = 0xFF17
+
+        .NR23           = 0x18  ; Sound register
+        rNR23           = 0xFF18
+        rAUD2LOW        = 0xFF18
+
+        .NR24           = 0x19  ; Sound register
+        rNR24           = 0xFF19
+        rAUD2HIGH       = 0xFF19
+
+        .NR30           = 0x1A  ; Sound register
+        rNR30           = 0xFF1A
+        rAUD3ENA        = 0xFF1A
+
+        .NR31           = 0x1B  ; Sound register
+        rNR31           = 0xFF1B
+        rAUD3LEN        = 0xFF1B
+
+        .NR32           = 0x1C  ; Sound register
+        rNR32           = 0xFF1C
+        rAUD3LEVEL      = 0xFF1C
+
+        .NR33           = 0x1D  ; Sound register
+        rNR33           = 0xFF1D
+        rAUD3LOW        = 0xFF1D
+
+        .NR34           = 0x1E  ; Sound register
+        rNR34           = 0xFF1E
+        rAUD3HIGH       = 0xFF1E
+
+        .NR41           = 0x20  ; Sound register
+        rNR41           = 0xFF20
+        rAUD4LEN        = 0xFF20
+
+        .NR42           = 0x21  ; Sound register
+        rNR42           = 0xFF21
+        rAUD4ENV        = 0xFF21
+
+        .NR43           = 0x22  ; Sound register
+        rNR43           = 0xFF22
+        rAUD4POLY       = 0xFF22
+
+        .NR44           = 0x23  ; Sound register
+        rNR44           = 0xFF23
+        rAUD4GO         = 0xFF23
+
+        .NR50           = 0x24  ; Sound register
+        rNR50           = 0xFF24
+        rAUDVOL         = 0xFF24
+
+        AUDVOL_VIN_LEFT  = 0b10000000 ; SO2
+        AUDVOL_VIN_RIGHT = 0b00001000 ; SO1
+
+        .NR51           = 0x25  ; Sound register
+        rNR51           = 0xFF25
+        rAUDTERM        = 0xFF25
+
+        AUDTERM_4_LEFT  = 0b10000000
+        AUDTERM_3_LEFT  = 0b01000000
+        AUDTERM_2_LEFT  = 0b00100000
+        AUDTERM_1_LEFT  = 0b00010000
+        AUDTERM_4_RIGHT = 0b00001000
+        AUDTERM_3_RIGHT = 0b00000100
+        AUDTERM_2_RIGHT = 0b00000010
+        AUDTERM_1_RIGHT = 0b00000001
+
+        .NR52           = 0x26  ; Sound register
+        rNR52           = 0xFF26
+        rAUDENA         = 0xFF26
+
+        AUDENA_ON       = 0b10000000
+        AUDENA_OFF      = 0b00000000  ; sets all audio regs to 0!
+
+        .LCDC           = 0x40  ; LCD control
+        rLCDC           = 0xFF40
+
+        LCDCF_OFF       = 0b00000000 ; LCD Control Operation
+        LCDCF_ON        = 0b10000000 ; LCD Control Operation
+        LCDCF_WIN9800   = 0b00000000 ; Window Tile Map Display Select
+        LCDCF_WIN9C00   = 0b01000000 ; Window Tile Map Display Select
+        LCDCF_WINOFF    = 0b00000000 ; Window Display
+        LCDCF_WINON     = 0b00100000 ; Window Display
+        LCDCF_BG8800    = 0b00000000 ; BG & Window Tile Data Select
+        LCDCF_BG8000    = 0b00010000 ; BG & Window Tile Data Select
+        LCDCF_BG9800    = 0b00000000 ; BG Tile Map Display Select
+        LCDCF_BG9C00    = 0b00001000 ; BG Tile Map Display Select
+        LCDCF_OBJ8      = 0b00000000 ; OBJ Construction
+        LCDCF_OBJ16     = 0b00000100 ; OBJ Construction
+        LCDCF_OBJOFF    = 0b00000000 ; OBJ Display
+        LCDCF_OBJON     = 0b00000010 ; OBJ Display
+        LCDCF_BGOFF     = 0b00000000 ; BG Display
+        LCDCF_BGON      = 0b00000001 ; BG Display
+        LCDCF_B_ON      = 7
+        LCDCF_B_WIN9C00 = 6
+        LCDCF_B_WINON   = 5
+        LCDCF_B_BG8000  = 4
+        LCDCF_B_BG9C00  = 3
+        LCDCF_B_OBJ16   = 2
+        LCDCF_B_OBJON   = 1
+        LCDCF_B_BGON    = 0
+
+        .STAT           = 0x41  ; LCD status
+        rSTAT           = 0xFF41
+
+        STATF_LYC       = 0b01000000 ; LYC=LY Coincidence (Selectable)
+        STATF_MODE10    = 0b00100000 ; Mode 10
+        STATF_MODE01    = 0b00010000 ; Mode 01 (V-Blank)
+        STATF_MODE00    = 0b00001000 ; Mode 00 (H-Blank)
+        STATF_LYCF      = 0b00000100 ; Coincidence Flag
+        STATF_HBL       = 0b00000000 ; H-Blank
+        STATF_VBL       = 0b00000001 ; V-Blank
+        STATF_OAM       = 0b00000010 ; OAM-RAM is used by system
+        STATF_LCD       = 0b00000011 ; Both OAM and VRAM used by system
+        STATF_BUSY      = 0b00000010 ; When set, VRAM access is unsafe
+        STATF_B_LYC     = 6
+        STATF_B_MODE10  = 5
+        STATF_B_MODE01  = 4
+        STATF_B_MODE00  = 3
+        STATF_B_LYCF    = 2
+        STATF_B_VBL     = 0
+        STATF_B_OAM     = 1
+        STATF_B_BUSY    = 1
+
+        .SCY            = 0x42  ; Scroll Y
+        rSCY            = 0xFF42
+
+        .SCX            = 0x43  ; Scroll X
+        rSCX            = 0xFF43
+
+        .LY             = 0x44  ; LCDC Y-coordinate
+        rLY             = 0xFF44
+
+        .LYC            = 0x45  ; LY compare
+        rLYC            = 0xFF45
+
+        .DMA            = 0x46  ; DMA transfer
+        rDMA            = 0xFF46
+
+        .BGP            = 0x47  ; BG palette data
+        rBGP            = 0xFF47
+
+        .OBP0           = 0x48  ; OBJ palette 0 data
+        rOBP0           = 0xFF48
+
+        .OBP1           = 0x49  ; OBJ palette 1 data
+        rOBP1           = 0xFF49
+
+        .WY             = 0x4A  ; Window Y coordinate
+        rWY             = 0xFF4A
+
+        .WX             = 0x4B  ; Window X coordinate
+        rWX             = 0xFF4B
+
+        .KEY1           = 0x4D  ; CPU speed
+        rKEY1           = 0xFF4D
+        rSPD            = 0xFF4D
+
+        KEY1F_DBLSPEED  = 0b10000000 ; 0=Normal Speed, 1=Double Speed (R)
+        KEY1F_PREPARE   = 0b00000001 ; 0=No, 1=Prepare (R/W)
+
+        .VBK            = 0x4F  ; VRAM bank
+        rVBK            = 0xFF4F
+
+        .HDMA1          = 0x51  ; DMA control 1
+        rHDMA1          = 0xFF51
+
+        .HDMA2          = 0x52  ; DMA control 2
+        rHDMA2          = 0xFF52
+
+        .HDMA3          = 0x53  ; DMA control 3
+        rHDMA3          = 0xFF53
+
+        .HDMA4          = 0x54  ; DMA control 4
+        rHDMA4          = 0xFF54
+
+        .HDMA5          = 0x55  ; DMA control 5
+        rHDMA5          = 0xFF55
+
+        HDMA5F_MODE_GP  = 0b00000000 ; General Purpose DMA (W)
+        HDMA5F_MODE_HBL = 0b10000000 ; HBlank DMA (W)
+
+        HDMA5F_BUSY     = 0b10000000 ; 0=Busy (DMA still in progress), 1=Transfer complete (R)
+
+        .RP             = 0x56  ; IR port
+        rRP             = 0xFF56
+
+        RPF_ENREAD      = 0b11000000
+        RPF_DATAIN      = 0b00000010 ; 0=Receiving IR Signal, 1=Normal
+        RPF_WRITE_HI    = 0b00000001
+        RPF_WRITE_LO    = 0b00000000
+
+        .BCPS           = 0x68  ; BG color palette specification
+        rBCPS           = 0xFF68
+
+        BCPSF_AUTOINC   = 0b10000000 ; Auto Increment (0=Disabled, 1=Increment after Writing)
+
+        .BCPD           = 0x69  ; BG color palette data
+        rBCPD           = 0xFF69
+
+        .OCPS           = 0x6A  ; OBJ color palette specification
+        rOCPS           = 0xFF6A
+
+        OCPSF_AUTOINC   = 0b10000000 ; Auto Increment (0=Disabled, 1=Increment after Writing)
+
+        .OCPD           = 0x6B  ; OBJ color palette data
+        rOCPD           = 0xFF6B
+
+        .SVBK           = 0x70  ; WRAM bank
+        rSVBK           = 0xFF70
+        rSMBK           = 0xFF70
+
+        rPCM12          = 0xFF76
+
+        rPCM34          = 0xFF77
+
+        .IE             = 0xFF  ; Interrupt enable
+        rIE             = 0xFFFF
+
+        .VBL_IFLAG      = 0x01
+        .LCD_IFLAG      = 0x02
+        .TIM_IFLAG      = 0x04
+        .SIO_IFLAG      = 0x08
+        .JOY_IFLAG      = 0x10
+
+        IEF_HILO        = 0b00010000 ; Transition from High to Low of Pin number P10-P13
+        IEF_SERIAL      = 0b00001000 ; Serial I/O transfer end
+        IEF_TIMER       = 0b00000100 ; Timer Overflow
+        IEF_STAT        = 0b00000010 ; STAT
+        IEF_VBLANK      = 0b00000001 ; V-Blank
+
+        ;; Flags common to multiple sound channels
+
+        AUDLEN_DUTY_12_5 = 0b00000000 ; 12.5%
+        AUDLEN_DUTY_25   = 0b01000000 ; 25%
+        AUDLEN_DUTY_50   = 0b10000000 ; 50%
+        AUDLEN_DUTY_75   = 0b11000000 ; 75%
+
+        AUDENV_UP       = 0b00001000
+        AUDENV_DOWN     = 0b00000000
+
+        AUDHIGH_RESTART    = 0b10000000
+        AUDHIGH_LENGTH_ON  = 0b01000000
+        AUDHIGH_LENGTH_OFF = 0b00000000
+
+        ;; OAM related constants
+
+        OAM_COUNT       = 40  ; number of OAM entries in OAM RAM
+
+        OAMF_PRI        = 0b10000000 ; Priority
+        OAMF_YFLIP      = 0b01000000 ; Y flip
+        OAMF_XFLIP      = 0b00100000 ; X flip
+        OAMF_PAL0       = 0b00000000 ; Palette number; 0,1 (DMG)
+        OAMF_PAL1       = 0b00010000 ; Palette number; 0,1 (DMG)
+        OAMF_BANK0      = 0b00000000 ; Bank number; 0,1 (GBC)
+        OAMF_BANK1      = 0b00001000 ; Bank number; 0,1 (GBC)
+
+        OAMF_PALMASK    = 0b00000111 ; Palette (GBC)
+
+        OAMB_PRI        = 7 ; Priority
+        OAMB_YFLIP      = 6 ; Y flip
+        OAMB_XFLIP      = 5 ; X flip
+        OAMB_PAL1       = 4 ; Palette number; 0,1 (DMG)
+        OAMB_BANK1      = 3 ; Bank number; 0,1 (GBC)
+
+        ;; CPU detection
+        .DMG_TYPE       = 0x01 ; Original GB or Super GB
+        .MGB_TYPE       = 0xFF ; Pocket GB or Super GB 2
+        .CGB_TYPE       = 0x11 ; Color GB
+
+        ;; GBDK library screen modes
+
+        .G_MODE         = 0x01  ; Graphic mode
+        .T_MODE         = 0x02  ; Text mode (bit 2)
+        .T_MODE_OUT     = 0x02  ; Text mode output only
+        .T_MODE_INOUT   = 0x03  ; Text mode with input
+        .M_NO_SCROLL    = 0x04  ; Disables scrolling of the screen in text mode
+        .M_NO_INTERP    = 0x08  ; Disables special character interpretation
+
+        ;; Status codes for IO
+        .IO_IDLE        = 0x00
+        .IO_SENDING     = 0x01
+        .IO_RECEIVING   = 0x02
+        .IO_ERROR       = 0x04
+
+        ;; Type of IO data
+        .DT_IDLE        = 0x66
+        .DT_RECEIVING   = 0x55
+
+        ;; Table of routines for modes
+        .MODE_TABLE     = 0x01E0
+
+        ;; C related
+        ;; Overheap of a banked call.  Used for parameters
+        ;;  = ret + real ret + bank
+
+        .if .NEAR_CALLS
+        .BANKOV         = 2
+
+        .else
+        .BANKOV         = 6
+
+        .endif
+
+        .globl  __current_bank
+        .globl  __shadow_OAM_base
+
+        ;; Global variables
+        .globl  .mode
+
+        .globl  __cpu
+        .globl  __is_GBA
+
+        ;; Global routines
+;       .globl  .set_mode       ;; don't link mode.o by default
+
+        .globl  .reset
+
+        .globl  .display_off
+
+        .globl  .wait_vbl_done
+
+        ;; Interrupt routines
+        .globl  .add_VBL
+;       .globl  .add_LCD        ;; don't link LCD.o by default
+;       .globl  .add_TIM        ;; don't link TIM.o by default
+;       .globl  .add_SIO        ;; don't link serial.o by default
+;       .globl  .add_JOY        ;; don't link JOY.o by default
+
+        ;; Symbols defined at link time
+        .globl  .STACK
+        .globl  _shadow_OAM
+        .globl  .refresh_OAM
+
+        ;; Main user routine
+        .globl  _main
+
+        ;; Macro definitions
+
+.macro WAIT_STAT ?lbl
+lbl:    LDH     A, (.STAT)
+        AND     #STATF_BUSY     ; Check if in LCD modes 0 or 1
+        JR      NZ, lbl
+.endm
+
+.macro ADD_A_REG16 regH regL
+        ADD     regL
+        LD      regL, A
+        ADC     regH
+        SUB     regL
+        LD      regH, A
+.endm
+
+.macro SIGNED_ADD_A_REG16 regH regL ?lbl
+        ; If A is negative, we need to subtract 1 from upper byte of 16-bit value
+        BIT     7, A            ; set z if a signed bit is 0
+        JR      Z, lbl          ; if z is set jump to positive
+        dec     regH            ; if negative decrement upper byte
+lbl:
+        ADD_A_REG16     regH, regL
+.endm
+
+.macro SIGNED_SUB_A_REG16 regH regL ?lbl
+        ; negate A then add to 16-bit value
+        CPL
+        INC     A
+        SIGNED_ADD_A_REG16      regH, regL
+.endm
+
+.macro MUL_DE_BY_A_RET_HL ?lbl1 ?lbl2
+        ; Multiply DE by A, return result in HL; preserves: BC
+        LD      HL, #0
+lbl1:
+        SRL     A
+        JR      NC, lbl2
+        ADD     HL, DE
+lbl2:
+        SLA     E
+        RL      D
+        OR      A
+        JR      NZ, lbl1
+.endm
+
diff --git a/presets/gb/gb/sfr.sgb b/presets/gb/gb/sfr.sgb
new file mode 100644
index 00000000..64febc7a
--- /dev/null
+++ b/presets/gb/gb/sfr.sgb
@@ -0,0 +1,111 @@
+__VRAM        = 0x8000
+__VRAM8000    = 0x8000
+__VRAM8800    = 0x8800
+__VRAM9000    = 0x9000
+__SCRN0       = 0x9800
+__SCRN1       = 0x9C00
+__SRAM        = 0xA000
+__RAM         = 0xC000
+__RAMBANK     = 0xD000
+__OAMRAM      = 0xFE00
+__IO          = 0xFF00
+__AUD3WAVERAM = 0xFF30
+__HRAM        = 0xFF80
+
+.globl __VRAM, __VRAM8000, __VRAM8800, __VRAM9000, __SCRN0, __SCRN1, __SRAM, __RAM, __RAMBANK, __OAMRAM, __IO, __AUD3WAVERAM, __HRAM       
+
+_rRAMG        = 0x0000
+_rROMB0       = 0x2000
+_rROMB1       = 0x3000
+_rRAMB        = 0x4000
+
+.globl _rRAMG, _rROMB0, _rROMB1, _rRAMB
+
+_P1_REG      = 0xFF00    ; Joystick: 1.1.P15.P14.P13.P12.P11.P10
+_SB_REG      = 0xFF01    ; Serial IO data buffer
+_SC_REG      = 0xFF02    ; Serial IO control register
+_DIV_REG     = 0xFF04    ; Divider register
+_TIMA_REG    = 0xFF05    ; Timer counter
+_TMA_REG     = 0xFF06    ; Timer modulo
+_TAC_REG     = 0xFF07    ; Timer control
+_IF_REG      = 0xFF0F    ; Interrupt flags: 0.0.0.JOY.SIO.TIM.LCD.VBL
+_NR10_REG    = 0xFF10    ; Sound register
+_NR11_REG    = 0xFF11    ; Sound register
+_NR12_REG    = 0xFF12    ; Sound register
+_NR13_REG    = 0xFF13    ; Sound register
+_NR14_REG    = 0xFF14    ; Sound register
+_NR21_REG    = 0xFF16    ; Sound register
+_NR22_REG    = 0xFF17    ; Sound register
+_NR23_REG    = 0xFF18    ; Sound register
+_NR24_REG    = 0xFF19    ; Sound register
+_NR30_REG    = 0xFF1A    ; Sound register
+_NR31_REG    = 0xFF1B    ; Sound register
+_NR32_REG    = 0xFF1C    ; Sound register
+_NR33_REG    = 0xFF1D    ; Sound register
+_NR34_REG    = 0xFF1E    ; Sound register
+_NR41_REG    = 0xFF20    ; Sound register
+_NR42_REG    = 0xFF21    ; Sound register
+_NR43_REG    = 0xFF22    ; Sound register
+_NR44_REG    = 0xFF23    ; Sound register
+_NR50_REG    = 0xFF24    ; Sound register
+_NR51_REG    = 0xFF25    ; Sound register
+_NR52_REG    = 0xFF26    ; Sound register
+_PCM_SAMPLE  = 0xFF30    ; PCM wave pattern
+_AUD3WAVE    = 0xFF30    ; PCM wave pattern
+_LCDC_REG    = 0xFF40    ; LCD control
+_STAT_REG    = 0xFF41    ; LCD status
+_SCY_REG     = 0xFF42    ; Scroll Y
+_SCX_REG     = 0xFF43    ; Scroll X
+_LY_REG      = 0xFF44    ; LCDC Y-coordinate
+_LYC_REG     = 0xFF45    ; LY compare
+_DMA_REG     = 0xFF46    ; DMA transfer
+_BGP_REG     = 0xFF47    ; BG palette data
+_OBP0_REG    = 0xFF48    ; OBJ palette 0 data
+_OBP1_REG    = 0xFF49    ; OBJ palette 1 data
+_WY_REG      = 0xFF4A    ; Window Y coordinate
+_WX_REG      = 0xFF4B    ; Window X coordinate
+_KEY1_REG    = 0xFF4D    ; CPU speed
+_VBK_REG     = 0xFF4F    ; VRAM bank
+_HDMA1_REG   = 0xFF51    ; DMA control 1
+_HDMA2_REG   = 0xFF52    ; DMA control 2
+_HDMA3_REG   = 0xFF53    ; DMA control 3
+_HDMA4_REG   = 0xFF54    ; DMA control 4
+_HDMA5_REG   = 0xFF55    ; DMA control 5
+_RP_REG      = 0xFF56    ; IR port
+_BCPS_REG    = 0xFF68    ; BG color palette specification
+_BCPD_REG    = 0xFF69    ; BG color palette data
+_OCPS_REG    = 0xFF6A    ; OBJ color palette specification
+_OCPD_REG    = 0xFF6B    ; OBJ color palette data
+_SVBK_REG    = 0xFF70    ; WRAM bank
+_PCM12_REG   = 0xFF76    ; Sound channel 1&2 PCM amplitude (R)
+_PCM34_REG   = 0xFF77    ; Sound channel 3&4 PCM amplitude (R)
+_IE_REG      = 0xFFFF    ; Interrupt enable
+
+.globl _P1_REG
+.globl _SB_REG, _SC_REG
+.globl _DIV_REG
+.globl _TIMA_REG, _TMA_REG, _TAC_REG
+.globl _IF_REG
+.globl _NR10_REG, _NR11_REG, _NR12_REG, _NR13_REG, _NR14_REG
+.globl _NR21_REG, _NR22_REG, _NR23_REG, _NR24_REG
+.globl _NR30_REG, _NR31_REG, _NR32_REG, _NR33_REG, _NR34_REG
+.globl _NR41_REG, _NR42_REG, _NR43_REG, _NR44_REG
+.globl _NR50_REG, _NR51_REG, _NR52_REG
+.globl _PCM_SAMPLE, _AUD3WAVE
+.globl _LCDC_REG
+.globl _STAT_REG
+.globl _SCY_REG, _SCX_REG
+.globl _LY_REG, _LYC_REG
+.globl _DMA_REG
+.globl _BGP_REG
+.globl _OBP0_REG, _OBP1_REG
+.globl _WY_REG, _WX_REG
+.globl _KEY1_REG
+.globl _VBK_REG
+.globl _HDMA1_REG, _HDMA2_REG, _HDMA3_REG, _HDMA4_REG, _HDMA5_REG
+.globl _RP_REG
+.globl _BCPS_REG, _BCPD_REG
+.globl _OCPS_REG, _OCPD_REG
+.globl _SVBK_REG
+.globl _PCM12_REG, _PCM34_REG
+.globl _IE_REG
\ No newline at end of file
diff --git a/presets/gb/hello.c b/presets/gb/hello.c
new file mode 100644
index 00000000..e6ca245c
--- /dev/null
+++ b/presets/gb/hello.c
@@ -0,0 +1,262 @@
+
+//#resource "gb/global.sgb"
+//#link "gb/crt0.sgb"
+//#link "gb/sfr.sgb"
+#include <stdint.h>
+#include "gb/types.h"
+#include "gb/hardware.h"
+#include "gb/gb.h"
+
+/* VRAM regions */
+#define TILE_DATA   ((volatile uint8_t *)0x8000)  /* 384 tiles × 16 bytes  */
+#define BG_MAP      ((volatile uint8_t *)0x9800)  /* 32×32 tile map        */
+
+/* ---------------------------------------------------------------------------
+ * wait_vblank
+ * Spin until LY reaches 144 (start of V-Blank period).
+ * ------------------------------------------------------------------------- */
+static void wait_vblank(void)
+{
+    while (rLY != 144);
+}
+
+/* ---------------------------------------------------------------------------
+ * Minimal 8×8 ASCII font (space 0x20 … 'Z' 0x5A)
+ * Each character is 8 bytes, one byte per row, MSB = leftmost pixel.
+ * Only printable upper-case letters, digits, space, and basic punctuation
+ * are included.
+ * ------------------------------------------------------------------------- */
+static const uint8_t font[][8] = {
+    /* 0x20  ' ' */ {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},
+    /* 0x21  '!' */ {0x18,0x18,0x18,0x18,0x00,0x00,0x18,0x00},
+    /* 0x22  '"' */ {0x6C,0x6C,0x00,0x00,0x00,0x00,0x00,0x00},
+    /* 0x23  '#' */ {0x6C,0x6C,0xFE,0x6C,0xFE,0x6C,0x6C,0x00},
+    /* 0x24  '$' */ {0x18,0x7E,0xC0,0x7C,0x06,0xFC,0x18,0x00},
+    /* 0x25  '%' */ {0xC6,0xCC,0x18,0x30,0x66,0xC6,0x00,0x00},
+    /* 0x26  '&' */ {0x38,0x6C,0x68,0x76,0xDC,0xCC,0x76,0x00},
+    /* 0x27  ''' */ {0x18,0x18,0x30,0x00,0x00,0x00,0x00,0x00},
+    /* 0x28  '(' */ {0x0C,0x18,0x30,0x30,0x30,0x18,0x0C,0x00},
+    /* 0x29  ')' */ {0x30,0x18,0x0C,0x0C,0x0C,0x18,0x30,0x00},
+    /* 0x2A  '*' */ {0x00,0x66,0x3C,0xFF,0x3C,0x66,0x00,0x00},
+    /* 0x2B  '+' */ {0x00,0x18,0x18,0x7E,0x18,0x18,0x00,0x00},
+    /* 0x2C  ',' */ {0x00,0x00,0x00,0x00,0x00,0x18,0x18,0x30},
+    /* 0x2D  '-' */ {0x00,0x00,0x00,0x7E,0x00,0x00,0x00,0x00},
+    /* 0x2E  '.' */ {0x00,0x00,0x00,0x00,0x00,0x18,0x18,0x00},
+    /* 0x2F  '/' */ {0x06,0x0C,0x18,0x30,0x60,0xC0,0x00,0x00},
+    /* 0x30  '0' */ {0x7C,0xC6,0xCE,0xD6,0xE6,0xC6,0x7C,0x00},
+    /* 0x31  '1' */ {0x18,0x38,0x18,0x18,0x18,0x18,0x7E,0x00},
+    /* 0x32  '2' */ {0x7C,0xC6,0x06,0x1C,0x70,0xC6,0xFE,0x00},
+    /* 0x33  '3' */ {0x7C,0xC6,0x06,0x3C,0x06,0xC6,0x7C,0x00},
+    /* 0x34  '4' */ {0x1C,0x3C,0x6C,0xCC,0xFE,0x0C,0x1E,0x00},
+    /* 0x35  '5' */ {0xFE,0xC0,0xF8,0x0C,0x06,0xCC,0x78,0x00},
+    /* 0x36  '6' */ {0x38,0x60,0xC0,0xF8,0xCC,0xCC,0x78,0x00},
+    /* 0x37  '7' */ {0xFE,0xC6,0x0C,0x18,0x30,0x30,0x30,0x00},
+    /* 0x38  '8' */ {0x78,0xCC,0xCC,0x78,0xCC,0xCC,0x78,0x00},
+    /* 0x39  '9' */ {0x78,0xCC,0xCC,0x7C,0x0C,0x18,0x70,0x00},
+    /* 0x3A  ':' */ {0x00,0x18,0x18,0x00,0x18,0x18,0x00,0x00},
+    /* 0x3B  ';' */ {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},
+    /* 0x3C  '<' */ {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},
+    /* 0x3D  '=' */ {0x00,0x00,0x7E,0x00,0x7E,0x00,0x00,0x00},
+    /* 0x3E  '>' */ {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},
+    /* 0x3F  '?' */ {0x7C,0xC6,0x06,0x1C,0x18,0x00,0x18,0x00},
+    /* 0x40  '@' */ {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},
+    /* 0x41  'A' */ {0x10,0x38,0x6C,0xC6,0xFE,0xC6,0xC6,0x00},
+    /* 0x42  'B' */ {0xFC,0xC6,0xC6,0xFC,0xC6,0xC6,0xFC,0x00},
+    /* 0x43  'C' */ {0x78,0xCC,0xC0,0xC0,0xC0,0xCC,0x78,0x00},
+    /* 0x44  'D' */ {0xF8,0xCC,0xC6,0xC6,0xC6,0xCC,0xF8,0x00},
+    /* 0x45  'E' */ {0xFE,0xC0,0xC0,0xF8,0xC0,0xC0,0xFE,0x00},
+    /* 0x46  'F' */ {0xFE,0xC0,0xC0,0xF8,0xC0,0xC0,0xC0,0x00},
+    /* 0x47  'G' */ {0x78,0xCC,0xC0,0xDE,0xCC,0xCC,0x7E,0x00},
+    /* 0x48  'H' */ {0xC6,0xC6,0xC6,0xFE,0xC6,0xC6,0xC6,0x00},
+    /* 0x49  'I' */ {0x7E,0x18,0x18,0x18,0x18,0x18,0x7E,0x00},
+    /* 0x4A  'J' */ {0x1E,0x06,0x06,0x06,0xC6,0xC6,0x7C,0x00},
+    /* 0x4B  'K' */ {0xC6,0xCC,0xD8,0xF0,0xD8,0xCC,0xC6,0x00},
+    /* 0x4C  'L' */ {0xC0,0xC0,0xC0,0xC0,0xC0,0xC0,0xFE,0x00},
+    /* 0x4D  'M' */ {0xC6,0xEE,0xFE,0xFE,0xD6,0xC6,0xC6,0x00},
+    /* 0x4E  'N' */ {0xC6,0xE6,0xF6,0xDE,0xCE,0xC6,0xC6,0x00},
+    /* 0x4F  'O' */ {0x78,0xCC,0xCC,0xCC,0xCC,0xCC,0x78,0x00},
+    /* 0x50  'P' */ {0xFC,0xC6,0xC6,0xFC,0xC0,0xC0,0xC0,0x00},
+    /* 0x51  'Q' */ {0x78,0xCC,0xCC,0xCC,0xEC,0x78,0x1C,0x00},
+    /* 0x52  'R' */ {0xFC,0xC6,0xC6,0xFC,0xD8,0xCC,0xC6,0x00},
+    /* 0x53  'S' */ {0x7C,0xC6,0xC0,0x7C,0x06,0xC6,0x7C,0x00},
+    /* 0x54  'T' */ {0x7E,0x18,0x18,0x18,0x18,0x18,0x18,0x00},
+    /* 0x55  'U' */ {0xC6,0xC6,0xC6,0xC6,0xC6,0xC6,0x7C,0x00},
+    /* 0x56  'V' */ {0xC6,0xC6,0xC6,0xC6,0x6C,0x38,0x10,0x00},
+    /* 0x57  'W' */ {0xC6,0xC6,0xD6,0xFE,0xFE,0xEE,0xC6,0x00},
+    /* 0x58  'X' */ {0xC6,0x6C,0x38,0x38,0x38,0x6C,0xC6,0x00},
+    /* 0x59  'Y' */ {0x66,0x66,0x66,0x3C,0x18,0x18,0x18,0x00},
+    /* 0x5A  'Z' */ {0xFE,0x06,0x0C,0x18,0x30,0x60,0xFE,0x00},
+};
+#define FONT_FIRST 0x20
+#define FONT_LAST  0x5A
+#define FONT_COUNT (FONT_LAST - FONT_FIRST + 1)
+
+/* ---------------------------------------------------------------------------
+ * load_font
+ * Write font glyphs into VRAM tile data.
+ * The Game Boy tile format stores two bits per pixel across two bitplanes;
+ * for a 2-colour font we set both bitplanes to the same byte so colour
+ * index 3 (black) is used for set pixels and 0 (white) for clear pixels.
+ * Tile 0 is reserved as blank; glyphs start at tile 1.
+ * ------------------------------------------------------------------------- */
+static void load_font(void)
+{
+    uint8_t t, r;
+    /* Tile 0: blank (already zeroed by boot ROM, but be explicit) */
+    for (r = 0; r < 16; r++) {
+        TILE_DATA[r] = 0x00;
+    }
+    /* Tiles 1 … FONT_COUNT: one tile per glyph */
+    for (t = 0; t < FONT_COUNT; t++) {
+        volatile uint8_t *dst = TILE_DATA + (t + 1) * 16;
+        for (r = 0; r < 8; r++) {
+            uint8_t row = font[t][r];
+            dst[r * 2]     = row;   /* bitplane 0 */
+            dst[r * 2 + 1] = row;   /* bitplane 1 — same → colour index 3 */
+        }
+    }
+}
+
+/* ---------------------------------------------------------------------------
+ * put_char / put_str
+ * Write a tile index into the BG map at tile position (col, row).
+ * Tile index = (ascii - FONT_FIRST) + 1, clamped to blank for unknowns.
+ * ------------------------------------------------------------------------- */
+static void put_char(uint8_t col, uint8_t row, char c)
+{
+    uint8_t tile;
+    if (c >= FONT_FIRST && c <= FONT_LAST)
+        tile = (uint8_t)(c - FONT_FIRST) + 1;
+    else
+        tile = 0;  /* blank */
+    BG_MAP[row * 32u + col] = tile;
+}
+
+static void put_str(uint8_t col, uint8_t row, const char *s)
+{
+    while (*s) {
+        put_char(col++, row, *s++);
+    }
+}
+
+/* ---------------------------------------------------------------------------
+ * clear_map
+ * Fill the visible 20×18 region of the BG map with tile 0 (blank).
+ * ------------------------------------------------------------------------- */
+static void clear_map(void)
+{
+    uint8_t r, c;
+    for (r = 0; r < 18; r++)
+        for (c = 0; c < 20; c++)
+            BG_MAP[r * 32u + c] = 0;
+}
+
+/* ---------------------------------------------------------------------------
+ * read_joypad
+ * Returns the buttons currently held.
+ * Bits: 7=start 6=select 5=B 4=A (action keys, P14 low)
+ *       3=down  2=up     1=left 0=right (d-pad, P15 low)
+ * We return a simple mask where bit 7 = START, bit 0 = RIGHT.
+ * ------------------------------------------------------------------------- */
+#define BTN_START  (1 << 7)
+#define BTN_SELECT (1 << 6)
+#define BTN_B      (1 << 5)
+#define BTN_A      (1 << 4)
+#define BTN_DOWN   (1 << 3)
+#define BTN_UP     (1 << 2)
+#define BTN_LEFT   (1 << 1)
+#define BTN_RIGHT  (1 << 0)
+
+static uint8_t read_joypad(void)
+{
+    uint8_t result = 0;
+
+    /* Read action buttons (P14 = 0) */
+    P1_REG = 0x20;
+    P1_REG; P1_REG;            /* dummy reads for settling */
+    result |= ((~P1_REG) & 0x0F) << 4;
+
+    /* Read d-pad (P15 = 0) */
+    P1_REG = 0x10;
+    P1_REG; P1_REG;
+    result |= ((~P1_REG) & 0x0F);
+
+    P1_REG = 0x30;           /* deselect both */
+    return result;
+}
+
+/* ---------------------------------------------------------------------------
+ * main
+ * ------------------------------------------------------------------------- */
+void main(void)
+{
+    uint8_t prev_joy = 0;
+    uint8_t curr_joy = 0;
+    uint8_t screen   = 0;
+
+    /* --- Disable LCD so we can safely write to VRAM --- */
+    wait_vblank();
+    LCDC_REG = 0x00;
+
+    /* --- Load font, clear map, set palette --- */
+    load_font();
+    clear_map();
+
+    /*
+     * Palette: 0=white, 1=light grey, 2=dark grey, 3=black
+     * BGP format: bits[1:0]=colour0, bits[3:2]=colour1, etc.
+     * 0xE4 = 11 10 01 00 → shade 0→0,1→1,2→2,3→3 (standard)
+     */
+    BGP_REG = 0xE4;
+    SCX_REG = 0;
+    SCY_REG = 0;
+
+    /* --- Draw initial screen --- */
+    put_str(3, 1, "GAME BOY DEMO");
+    put_str(2, 3, "HELLO, WORLD!");
+    put_str(1, 5, "BUILT WITH SDCC");
+    put_str(0, 16, "PRESS START");
+
+    /*
+     * Re-enable LCD:
+     *   Bit 7: LCD on
+     *   Bit 4: tile data at 0x8000 (unsigned indexing)
+     *   Bit 0: BG enable
+     */
+    LCDC_REG = LCDCF_ON | LCDCF_WIN9800 | LCDCF_BG8000 | LCDCF_B_BGON;
+
+    /* --- Main loop --- */
+    for (;;) {
+        wait_vblank();
+
+        curr_joy = read_joypad();
+
+        /* Edge detect START */
+        if ((curr_joy & BTN_START) && !(prev_joy & BTN_START)) {
+            if (screen == 0) {
+                /* Switch to second screen */
+                wait_vblank();
+                LCDC_REG = 0x00;
+                clear_map();
+                put_str(1, 4, "YOU PRESSED START!");
+                put_str(3, 6, "HAVE FUN :)");
+                put_str(0, 16, "PRESS START AGAIN");
+    LCDC_REG = LCDCF_ON | LCDCF_B_BGON;
+                screen = 1;
+            } else {
+                /* Back to title */
+                wait_vblank();
+                LCDC_REG = 0x00;
+                clear_map();
+                put_str(3, 1, "GAME BOY DEMO");
+                put_str(2, 3, "HELLO, WORLD!");
+                put_str(1, 5, "BUILT WITH SDCC");
+                put_str(0, 16, "PRESS START");
+    LCDC_REG = LCDCF_ON | LCDCF_B_BGON;
+                screen = 0;
+            }
+        }
+
+        prev_joy = curr_joy;
+    }
+}
diff --git a/src/worker/platforms.ts b/src/worker/platforms.ts
index a1591d71..60b69826 100644
--- a/src/worker/platforms.ts
+++ b/src/worker/platforms.ts
@@ -366,9 +366,14 @@ export var PLATFORM_PARAMS = {
       arch: 'gbz80',
       code_start: 0x0,
       rom_size: 0x8000,
-      data_start: 0xc000,
-      data_size: 0x2000,
+      data_start: 0xc0a0,
+      data_size: 0x1f60,
       stack_end: 0xe000,
+      extra_link_args: [
+        '-g', '_shadow_OAM=0xC000',
+        '-g', '.STACK=0xE000',
+        '-g', '.refresh_OAM=0xFF80',
+      ],
       wiz_sys_type: 'gb',
       wiz_inc_dir: 'gb',
     },