1 # Draw pixels in response to keyboard events, starting from the top-left
 2 # and in raster order.
 3 #
 4 # To run, first prepare a realistically sized disk image:
 5 #   dd if=/dev/zero of=disk.img count=20160  # 512-byte sectors, so 10MB
 6 # Load the program on the disk image:
 7 #   cat baremetal/boot.hex baremetal/ex3.hex  |./bootstrap run apps/hex  > a.bin
 8 #   dd if=a.bin of=disk.img conv=notrunc
 9 # To run:
10 #   qemu-system-i386 disk.img
11 # Or:
12 #   bochs -f baremetal/boot.bochsrc  # boot.bochsrc loads disk.img
13 
14 # main:  (address 0x8800)
15 
16 # eax <- LFB
17 8b  # copy *rm32 to r32
18   05  # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32
19   28 7f 00 00 # disp32 [label]
20 
21 # var read index/ecx: byte = 0
22 31 c9  # ecx <- xor ecx;  11/direct 001/r32/ecx 001/rm32/ecx
23 
24 # $loop:
25   # CL = *read index
26   8a  # copy m8 at r32 to r8
27     0d  # 00/mod/indirect 001/r8/cl 101/rm32/use-disp32
28     cf 7d 00 00  # disp32 [label]
29   # CL = *(keyboard buffer + ecx)
30   8a  # copy m8 at r32 to r8
31     89  # 10/mod/*+disp32 001/r8/cl 001/rm32/ecx
32     d0 7d 00 00  # disp32 [label]
33   # if (CL == 0) loop (spin loop)
34   80
35     f9  # 11/mod/direct 111/subop/compare 001/rm8/CL
36     00  # imm8
37   74 ef  # loop -17 [label]
38 # offset 0x19:
39   # otherwise increment read index
40   fe  # increment byte
41     05  # 00/mod/indirect 000/subop/increment 101/rm32/use-disp32
42     cf 7d 00 00  # disp32 [label]
43   # clear top nibble of index (keyboard buffer is circular)
44   80  # and byte
45     25  # 00/mod/indirect 100/subop/and 101/rm32/use-disp32
46     cf 7d 00 00  # disp32 [label]
47     0f  # imm8
48   # print a pixel in fluorescent green
49   c6  # copy imm8 to m8 at rm32
50     00  # 00/mod/indirect 000/subop 000/rm32/eax
51     31  # imm32
52   40  # increment eax
53   eb dc # loop -36 [label]
54 
55 # $break:
56 e9 fb ff ff ff  # hang indefinitely
57 
58 # vim:ft=subx