path: root/ex3.hex

# Draw pixels in response to keyboard events, starting from the top-left
# and in raster order.
#
# To run, first prepare a realistically sized disk image:
#   dd if=/dev/zero of=disk.img count=20160  # 512-byte sectors, so 10MB
# Load the program on the disk image:
#   cat baremetal/boot.hex baremetal/ex3.hex  |bootstrap/bootstrap run hex  > a.bin
#   dd if=a.bin of=disk.img conv=notrunc
# To run:
#   qemu-system-i386 disk.img
# Or:
#   bochs -f baremetal/boot.bochsrc  # boot.bochsrc loads disk.img

# main:  (address 0x9000)

# eax <- LFB
8b  # copy *rm32 to r32
  05  # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32
  28 81 00 00 # disp32 [label]

# var read index/ecx: byte = 0
31 c9  # ecx <- xor ecx;  11/direct 001/r32/ecx 001/rm32/ecx

# $loop:
  # CL = *read index
  8a  # copy m8 at r32 to r8
    0d  # 00/mod/indirect 001/r8/cl 101/rm32/use-disp32
    cc 7d 00 00  # disp32 [label]
  # CL = *(keyboard buffer + ecx)
  8a  # copy m8 at r32 to r8
    89  # 10/mod/*+disp32 001/r8/cl 001/rm32/ecx
    d0 7d 00 00  # disp32 [label]
  # if (CL == 0) loop (spin loop)
  80
    f9  # 11/mod/direct 111/subop/compare 001/rm8/CL
    00  # imm8
  74 ef  # loop -17 [label]
# offset 0x19:
  # otherwise increment read index
  fe  # increment byte
    05  # 00/mod/indirect 000/subop/increment 101/rm32/use-disp32
    cc 7d 00 00  # disp32 [label]
  # clear top nibble of index (keyboard buffer is circular)
  80  # and byte
    25  # 00/mod/indirect 100/subop/and 101/rm32/use-disp32
    cc 7d 00 00  # disp32 [label]
    0f  # imm8
  # print a pixel in fluorescent green
  c6  # copy imm8 to m8 at rm32
    00  # 00/mod/indirect 000/subop 000/rm32/eax
    31  # imm32
  40  # increment eax
  eb dc # loop -36 [label]

# $break:
e9 fb ff ff ff  # hang indefinitely

# vim:ft=subx