path: root/baremetal/101screen.subx

# Primitives for screen control.
#
# We need to do this in machine code because Mu doesn't have global variables
# yet (for the start of video memory).

== code

pixel-on-real-screen:  # x: int, y: int, color: int
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    50/push-eax
    51/push-ecx
    # bounds checks
    8b/-> *(ebp+8) 0/r32/eax
    3d/compare-eax-and 0/imm32
    7c/jump-if-< $pixel-on-real-screen:end/disp8
    3d/compare-eax-and 0x400/imm32/screen-width=1024
    7d/jump-if->= $pixel-on-real-screen:end/disp8
    8b/-> *(ebp+0xc) 0/r32/eax
    3d/compare-eax-and 0/imm32
    7c/jump-if-< $pixel-on-real-screen:end/disp8
    3d/compare-eax-and 0x300/imm32/screen-height=768
    7d/jump-if->= $pixel-on-real-screen:end/disp8
    # eax = y*1024 + x
    8b/-> *(ebp+0xc) 0/r32/eax
    c1/shift 4/subop/left %eax 0xa/imm8
    03/add-> *(ebp+8) 0/r32/eax
    # eax += location of frame buffer
    03/add-> *0x7f28 0/r32/eax  # unsafe
    # *eax = color
    8b/-> *(ebp+0x10) 1/r32/ecx
    88/byte<- *eax 1/r32/CL
$pixel-on-real-screen:end:
    # . restore registers
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return