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65816 assembly routine to set a color in the Apple IIgs palette table without using temp variables (uses stack addition)
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| mx %00 ; full 16-bit registers | |
| lda #$0000 ; palette 0 | |
| ldx #$0001 ; color 1 | |
| ldy #$033F ; blue (r=3,g=3,b=F) | |
| jsr SHR_SETPALIDXCOLOR | |
| jmp Quit ; or whatever comes next | |
| * Set color in IIgs palette table | |
| * call with pal=A, color index=X, 0rgb=Y | |
| SHR_SETPALIDXCOLOR asl ; a<<5 (a*$20) for palette offset | |
| asl | |
| asl | |
| asl | |
| asl | |
| pha ; push pal offset | |
| txa ; NOTE NO CLC BECAUSE WE EXPECT 0 FOR HIGH BITS OF X | |
| asl ; a*2 for color index offset (word) | |
| adc 1,s ; add to pal offset | |
| tax ; back to index register now that we have final offset | |
| pla ; pull ephemeral variable | |
| tya ; get our color rgb value to save | |
| stal $e19e00,x ; store in palette area | |
| rts | |
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