mirror of
https://github.com/RevCurtisP/C02.git
synced 2024-11-30 23:50:04 +00:00
72 lines
2.4 KiB
Plaintext
72 lines
2.4 KiB
Plaintext
;Atart 2600 Program Stub - C02 Assembly Language File
|
|
|
|
;Preset Total Number of Kernal Lines
|
|
KNLLNS SET 192 ;192 Lines on NTSC Television
|
|
|
|
;Variables Used By Other Libraries
|
|
TEMP0 EQU $E0 ;Temporary Storage
|
|
TEMP1 EQU $E1
|
|
TEMP2 EQU $E2
|
|
TEMP3 EQU $E3
|
|
|
|
;Initialization Code
|
|
; From CLEAN_START macro by Andrew Davie
|
|
; Standardised STArt-up code, clears STAck, all TIA registers and RAM to 0
|
|
; Sets STAck pointer to $FF, and all registers to 0
|
|
; Sets decimal mode off, sets interrupt flag (kind of un-necessary)
|
|
; Use as very first section of code on boot (ie: at reset)
|
|
; Code written to minimise total ROM usage - uses weird 6502 knowledge :)
|
|
START: SEI
|
|
CLD
|
|
LDX #0
|
|
TXA
|
|
TAY
|
|
STARTC: DEX
|
|
TXS
|
|
PHA
|
|
BNE STARTC
|
|
|
|
JSR SETUP ;Execute Setup Code
|
|
|
|
MAIN: LDA #2 ;Vertical Sync On Bit Mask
|
|
LDX #49 ;41 scanlines * 76 cycles / 64
|
|
STA WSYNC ;Wait for SYNC (halts CPU until end of scanline)
|
|
STA VSYNC ;Turn On Vertical Sync
|
|
STX TIM64T ;Set timer to go off in 41 scanlines
|
|
STA WSYNC ;First Scanline of VSYNC
|
|
STA WSYNC ;Second Scanline of VSYNC
|
|
LDA #0 ;Vertical Sync Off Bit Mask
|
|
STA WSYNC ;Third Scanline of VSYNC
|
|
STA VSYNC ;Turn Off Vertical Sync
|
|
JSR VRTBLK ;Execute Vertical Blank Code
|
|
STA WSYNC ;Wait For Horizontal Sync
|
|
STA HMOVE ;Execute Horizontal Moves
|
|
JSR CLRHMX ;Clear Horizontal Move Registers
|
|
WTVBLK: STA WSYNC ;Wait for End of Vertical Blank
|
|
LDA INTIM ;Check Timer
|
|
BNE WTVBLK ;Loop If Not Zero
|
|
STA VBLANK ;Turn off Vertical Blank
|
|
JSR KERNEL ;Execute Kernel Code
|
|
STA WSYNC ;Wait for End of Scanline
|
|
LDA #2 ;Vertical Blank On Bit Mask
|
|
STA VBLANK ;Turn on Vertical Blank
|
|
LDA #32 ;27 scanlines * 76 cycles / 64
|
|
STA TIM64T ;Set Timer for 27 scanlines
|
|
JSR OVRSCN ;Execute Overscan Code
|
|
WTOSCN: STA WSYNC ;Wait for End of Overscan
|
|
LDA INTIM ;Check Time
|
|
BNE WTOSCN ;Loop If Not Zero
|
|
BEQ MAIN ;Loop Back to Vertical Sync
|
|
|
|
IRQBRK: RTI ;Catch 6502 Break Instructions
|
|
|
|
CLRHMR: NOP ; 2 8 (+6 for calling JSR)
|
|
JSR CLRHMX ;12 20
|
|
LDA #0 ; 2 22 - Clear Horizontal Move Registers
|
|
LDX #4 ; 2 24
|
|
CLRHML STA HMP0,X ; - Safe to Clear 24 Cycles after HMOVE
|
|
DEX
|
|
BPL CLRHML
|
|
CLRHMX: RTS
|
|
|