https://github.com/akkartik/mu/blob/main/101screen.subx

     1 # Primitives for screen control.
     2 #
     3 # We need to do this in machine code because Mu doesn't have global variables
     4 # yet (for the start of video memory).
     5
     6 == code
     7
     8 pixel-on-real-screen:  # x: int, y: int, color: int
     9     # . prologue
    10     55/push-ebp
    11     89/<- %ebp 4/r32/esp
    12     # . save registers
    13     50/push-eax
    14     51/push-ecx
    15     # bounds checks
    16     8b/-> *(ebp+8) 0/r32/eax
    17     3d/compare-eax-and 0/imm32
    18     7c/jump-if-< $pixel-on-real-screen:end/disp8
    19     3d/compare-eax-and 0x400/imm32/screen-width=1024
    20     7d/jump-if->= $pixel-on-real-screen:end/disp8
    21     8b/-> *(ebp+0xc) 0/r32/eax
    22     3d/compare-eax-and 0/imm32
    23     7c/jump-if-< $pixel-on-real-screen:end/disp8
    24     3d/compare-eax-and 0x300/imm32/screen-height=768
    25     7d/jump-if->= $pixel-on-real-screen:end/disp8
    26     # eax = y*1024 + x
    27     8b/-> *(ebp+0xc) 0/r32/eax
    28     c1/shift 4/subop/left %eax 0xa/imm8
    29     03/add-> *(ebp+8) 0/r32/eax
    30     # eax += location of frame buffer
    31     03/add-> *Video-memory-addr 0/r32/eax
    32     # *eax = color
    33     8b/-> *(ebp+0x10) 1/r32/ecx
    34     88/byte<- *eax 1/r32/CL
    35 $pixel-on-real-screen:end:
    36     # . restore registers
    37     59/pop-to-ecx
    38     58/pop-to-eax
    39     # . epilogue
    40     89/<- %esp 5/r32/ebp
    41     5d/pop-to-ebp
    42     c3/return