# check keyboard for a key
# return 0 on no keypress or unrecognized key
#
# We need to do this in machine code because Mu doesn't have global variables
# yet (for the keyboard buffer).

== code

read-key:  # kbd: (addr keyboard) -> result/eax: byte
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    51/push-ecx
    # result = 0
    b8/copy-to-eax 0/imm32
    # ecx = keyboard
    8b/-> *(ebp+8) 1/r32/ecx
    81 7/subop/compare %ecx 0/imm32
    {
      75/jump-if-!= break/disp8
      # var read/ecx: byte = keyboard buffer's read index
      8b/-> *0x802c 1/r32/CL  # keyboard-buffer-read
      # var next-key/eax: byte = *(keyboard buffer + ecx)
      8a/byte-> *(ecx+0x8030) 0/r32/AL  # keyboard-buffer-data
      # if (next-key != 0) lock and remove from keyboard buffer
      81 7/subop/compare %eax 0/imm32
      {
        74/jump-if-= break/disp8
        # TODO: add some instructions in this block to SubX if we ever want to
        # use bootstrap on baremetal programs
        fa/disable-interrupts
        c6 0/subop/copy-byte *(ecx+0x8030) 0/imm8  # keyboard-buffer-data
        ff 0/subop/increment *0x802c  # keyboard-buffer-read
        81 4/subop/and *0x802c 0xf/imm32  # keyboard-buffer-read
        fb/enable-interrupts
      }
      # return
      eb $read-key:end/disp8
    }
    # TODO: fake keyboard
$read-key:end:
    # . restore registers
    59/pop-to-ecx
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return