calc84maniac

@ void setPixel(void* buffer, unsigned x, unsigned y, unsigned color);
.global setPixel

@ Notes:
@  Only bottom 4 bits of color are used, the rest can be garbage.
@  This routine will not write pixels offscreen.
@  Cycle timing (not including function call):
@  * Offscreen pixel:        5 cycles
@  * Onscreen pixel, x%2==0: 16 cycles
@  * Onscreen pixel, x%2==1: 15 cycles
@  Code size: 68 bytes
setPixel:
        cmp r1,#320
        cmplo r2,#240
        bxhs lr
        add r2,r2,r2,lsl #2
        add r0,r0,r2,lsl #5
        movs r1,r1,lsr #1
        ldrb r2,[r0,r1]
        bcc 1f
        and r3,r3,#0x0F
        bic r2,r2,#0x0F
        orr r2,r2,r3
        strb r2,[r0,r1]
        bx lr
1:
        bic r2,r2,#0xF0
        orr r2,r2,r3,lsl #4
        strb r2,[r0,r1]
        bx lr