cc65/libsrc/lynx/bootldr.s

;
; Karri Kaksonen, 2011
;
; This bootloader creates a signed binary so that the Lynx will accept it.
;
	.include "lynx.inc"
	.include "extzp.inc"
	.import		__BLOCKSIZE__
	.export		__BOOTLDR__: absolute = 1


; ------------------------------------------------------------------------
; Bootloader

	.segment "BOOTLDR"
;**********************************
; Here is the bootloader in plaintext
; The idea is to make the smalles possible encrypted loader as decryption
; is very slow. The minimum size is 49 bytes plus a zero byte.
;**********************************
;	EXE = $f000
;
;	.org $0200
;
;	; 1. force Mikey to be in memory
;	stz MAPCTL
;
;	; 3. set ComLynx to open collector
;	lda #4          ; a = 00000100
;	sta SERCTL      ; set the ComLynx to open collector
;
;	; 4. make sure the ROM is powered on
;	lda #8          ; a = 00001000
;	sta IODAT       ; set the ROM power to on
;
;	; 5. read in secondary exe + 8 bytes from the cart and store it in $f000
;	ldx #0          ; x = 0
;	ldy #$97        ; y = secondary loader size (151 bytes)
;rloop1: lda RCART0     ; read a byte from the cart
;	sta EXE,X       ; EXE[X] = a
;	inx             ; x++
;	dey             ; y--
;	bne rloop1      ; loops until y wraps
;
;	; 6. jump to secondary loader
;	jmp EXE         ; run the secondary loader
;
;	.reloc
;**********************************
; After compilation, encryption and obfuscation it turns into this.
;**********************************
	.byte $ff, $30, $73, $35, $4a, $a8, $54, $ef 
	.byte $54, $20, $f5, $38, $f4, $35, $7e, $31 
	.byte $7a, $c3, $f6, $eb, $ee, $30, $e3, $e5 
	.byte $81, $91, $85, $bf, $4b, $d9, $cf, $80 
	.byte $5f, $54, $36, $b5, $8a, $b0, $50, $d6 
	.byte $38, $22, $3e, $c1, $01, $a6, $dd, $f5 
	.byte $4b, $5e, $6b, $21

;**********************************
; Now we have the secondary loader
;**********************************
	.org $f000
	; 1. Read in the 1st File-entry (main exe) in FileEntry
	ldx #$00
	ldy #8
rloop:	lda RCART0      ; read a byte from the cart
	sta _FileEntry,X ; EXE[X] = a
	inx
	dey
	bne rloop

	; 2. Set the block hardware to the main exe start
	lda	_FileStartBlock
	sta	_FileCurrBlock
	jsr	seclynxblock

	; 3. Skip over the block offset
	lda	_FileBlockOffset+1
        eor	#$FF
	tay
	lda	_FileBlockOffset
        eor	#$FF
	tax
	jsr	seclynxskip0

	; 4. Read in the main exe to RAM
	lda	_FileDestAddr
	ldx	_FileDestAddr+1
	sta     _FileDestPtr
	stx     _FileDestPtr+1
	lda     _FileFileLen+1
	eor	#$FF
	tay
	lda     _FileFileLen
	eor	#$FF
	tax
	jsr     seclynxread0

	; 5. Jump to start of the main exe code
	jmp	(_FileDestAddr)

;**********************************
; Skip bytes on bank 0
; X:Y count (EOR $FFFF)
;**********************************
seclynxskip0:
	inx
	bne @0
	iny
	beq exit
@0:	jsr secreadbyte0
	bra seclynxskip0

;**********************************
; Read bytes from bank 0
; X:Y count (EOR $ffff)
;**********************************
seclynxread0:
	inx
	bne @1
	iny
	beq exit
@1:	jsr secreadbyte0
	sta (_FileDestPtr)
	inc _FileDestPtr
	bne seclynxread0
	inc _FileDestPtr+1
	bra seclynxread0

;**********************************
; Read one byte from cartridge
;**********************************
secreadbyte0:
	lda RCART0
	inc _FileBlockByte
	bne exit
	inc _FileBlockByte+1
	bne exit

;**********************************
; Select a block 
;**********************************
seclynxblock:
	pha
	phx
	phy
	lda __iodat
	and #$fc
	tay
	ora #2
	tax
	lda _FileCurrBlock
	inc _FileCurrBlock
	sec
	bra @2
@0:	bcc @1
	stx IODAT
	clc
@1:	inx
	stx SYSCTL1
	dex
@2:	stx SYSCTL1
	rol
	sty IODAT
	bne @0
	lda __iodat
	sta IODAT
	stz _FileBlockByte
	lda #<($100-(>__BLOCKSIZE__))
	sta _FileBlockByte+1
	ply
	plx
	pla

exit:	rts

	.reloc