Posted By
Giuseppe
on 2010-11-09
16:51:12
|  Re: Assembly and interrupts: my first attempt. Help needed!
I post here the source code of my small program, so you guys can have a look at it without downloading the zip above:
; ** File: RASTER.ASM
; ** Version: v0.9 - 08/11/2010
; ** Author: Giuseppe Mignogna
;
; Code written in Assembly 6502 for the AS65 compiler.
; Load into Plus/4 emulator and run with "SYS 8192".
; This code is for studying purposes and it's not optimized at all!
; -- Macro macroSR ------------------------------
macroSR macro
; Save registers to stack
PHA
TXA
PHA
TYA
PHA
endm
; -----------------------------------------------
; -- Macro macroRR ------------------------------
macroRR macro
; Restore registers from stack
PLA
TAY
PLA
TAX
PLA
endm
; -----------------------------------------------
; -- Initializing -------------------------------
; Locate code at $2000
org $2000
; Set the first sprite to be processed
LDA #$00
STA spriteIndex
; Set the buffer as not ready
STA isBufferReady
; Modify the VBLANK vector to point at a custom routine
SEI
LDA #VBLANKIRQ & 255
STA $0312
LDA #VBLANKIRQ >> 8
STA $0313
CLI
; -----------------------------------------------
; -- Main ----------------------------------------
MAIN
; The MAIN routine is looped endlessley. It fills a screen buffer
; located at $3000 with characters (sprites) moving from right to
; left. Sprites data are defined into a table below. Some data is
; not used at the moment.
; Check if the buffer is ready to be copied into screen memory
LDA isBufferReady
BNE jumpOver02
; Buffer is not ready, then clear the screen buffer
LDA #$20
LDX #$00
erase01
STA $3000,X
STA $3100,X
STA $3200,X
STA $3300,X
DEX
BNE erase01
drawSprites
; Calculate spriteindex*8
LDA spriteIndex
ASL A
ASL A
ASL A
TAY
; Get screen address from X,Y coords stored into sprites table
JSR xy2addr
; Put sprite character into screen buffer
; Address is stored at $d2,$d3
LDX #$00
LDA sprites+4,y ; sprite character
STA ($D2,X)
; Manage delay
TYA
TAX
DEC sprites+6,x ; delay counter
BNE jumpOver01
; Restore delay value
LDA sprites+5,x ; original delay value
STA sprites+6,x
; Decrease X coord (sprite moves left)
DEC sprites,x
BNE jumpOver01
; Restore X coord
LDA #$27 ; column 39
STA sprites,x
jumpOver01
; Set spriteindex to next sprite
INC spriteIndex
LDA spriteIndex
CMP maxSprites
BNE drawSprites
; Buffer has been filled with sprites and now
; it's ready to be written into screen memory
; by the VBLANK routine. Meanwhile, the MAIN
; program will do nothing, besides checking the
; isBufferReady variable to be zero again.
LDA #$00
STA spriteIndex
LDA #$01
STA isBufferReady
jumpOver02
JMP MAIN
; ------------------------------------------------
; -- VBLANK routine ------------------------------
VBLANKIRQ
; Check if the buffer is ready to be copied into screen memory
LDA isBufferReady
BEQ jumpOver04
; Copy the buffer into screen memory
LDX #$00
copyBuffer
LDA $3000,X
STA $0c00,X
LDA $3100,X
STA $0d00,X
LDA $3200,X
STA $0e00,X
LDA $3300,X
STA $0f00,X
DEX
BNE copyBuffer
; Buffer now is not ready again and must be filled by the MAIN routine
LDA #$00
STA isBufferReady
jumpOver04
JMP $CE42
; ------------------------------------------------
; -- Subroutine xy2addr --------------------------
; Input: register Y = nr. sprite*8
; (each sprite has 8 bytes of data in the table)
xy2addr
macroSR ; save registers
; Calculate the memory address corresponding to the
; X and Y coordinates using a table that stores all the
; 25 addresses of the column zero
LDA sprites+1,y ; Y coord from sprite table
ASL A ; Y = Y * 2
TAX
LDA columnZero,x ; low byte address
STA $D2
INX
LDA columnZero,x ; high byte address
STA $D3
; Add X coord to address
LDA sprites,y ; X coord from sprite table
ADC $D2 ; add to low byte
STA $D2
BCC jumpOver03 ; if carry is clear, do nothing
INC $D3 ; on carry set, increase high byte (next page)
jumpOver03
macroRR ; restore registers
RTS
; ------------------------------------------------
; -- Data ---------------------------------------
columnZero
DW $3000, $3028, $3050, $3078, $30A0, $30C8, $30F0, $3118
DW $3140, $3168, $3190, $31B8, $31E0, $3208, $3230, $3258
DW $3280, $32A8, $32D0, $32F8, $3320, $3348, $3370, $3398
DW $33C0
maxSprites
DB $10
sprites
; x y ofs col chr dly dlv sts
DB $27,$02,$00,$00,$D1,$14,$14,$00
DB $27,$03,$00,$00,$D0,$03,$03,$00
DB $27,$04,$00,$00,$D0,$04,$04,$00
DB $27,$05,$00,$00,$D1,$07,$07,$00
DB $27,$06,$00,$00,$D0,$01,$01,$00
DB $27,$07,$00,$00,$D0,$02,$02,$00
DB $27,$08,$00,$00,$D1,$06,$06,$00
DB $27,$09,$00,$00,$D1,$01,$01,$00
DB $27,$0A,$00,$00,$D0,$14,$14,$00
DB $27,$0B,$00,$00,$D0,$03,$03,$00
DB $27,$0C,$00,$00,$D1,$04,$04,$00
DB $27,$0D,$00,$00,$D0,$07,$07,$00
DB $27,$0E,$00,$00,$D1,$01,$01,$00
DB $27,$0F,$00,$00,$D1,$02,$02,$00
DB $27,$10,$00,$00,$D0,$06,$06,$00
DB $27,$11,$00,$00,$D1,$01,$01,$00
spriteIndex
DB $00
isBufferReady
DB $00
; -----------------------------------------------
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