1 # check keyboard for a key
 2 # return 0 on no keypress or unrecognized key
 3 
 4 == code
 5 
 6 read-key:  # kbd: (addr keyboard) -> result/eax: byte
 7     # . prologue
 8     55/push-ebp
 9     89/<- %ebp 4/r32/esp
10     # . save registers
11     51/push-ecx
12     # result = 0
13     b8/copy-to-eax 0/imm32
14     # ecx = keyboard
15     8b/-> *(ebp+8) 1/r32/ecx
16     81 7/subop/compare %ecx 0/imm32
17     {
18       75/jump-if-!= break/disp8
19       # var read/ecx: byte = keyboard buffer's read index
20       8b/-> *0x7dcc 1/r32/CL  # keyboard-buffer-read
21       # var next-key/eax: byte = *(keyboard buffer + ecx)
22       8a/byte-> *(ecx+0x7dd0) 0/r32/AL  # keyboard-buffer-data
23       # if (next-key != 0) lock and remove from keyboard-buffer
24       81 7/subop/compare %eax 0/imm32
25       {
26         74/jump-if-= break/disp8
27         # TODO: add some instructions in this block to SubX if we ever want to
28         # use bootstrap on baremetal programs
29         fa/disable-interrupts
30         c6 0/subop/copy-byte *(ecx+0x7dd0) 0/imm8
31         ff 0/subop/increment *0x7dcc  # keyboard-buffer-read
32         81 4/subop/and *0x7dcc 0xf/imm32  # keyboard-buffer-read
33         fb/enable-interrupts
34       }
35       # return
36       eb $read-key:end/disp8
37     }
38     # TODO: fake keyboard
39 $read-key:end:
40     # . restore registers
41     59/pop-to-ecx
42     # . epilogue
43     89/<- %esp 5/r32/ebp
44     5d/pop-to-ebp
45     c3/return