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# 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
# Most keys correspond to their ASCII/Unicode values.
# TODO: Support for international keyboards and multi-byte Unicode.
#
# However there are some exceptions that have no assigned place in Unicode
# (and with good reason):
# 0x80 = left arrow ←
# 0x81 = down arrow ↓
# 0x82 = up arrow ↑
# 0x83 = right arrow →
# These code points are not used by Unicode and their semantics are agreed to
# be context-sensitive: https://en.wikipedia.org/wiki/C0_and_C1_control_codes#C1_controls.
# Mu cannibalizes them in yet another non-standard way.
read-key: # kbd: (addr keyboard) -> result/eax: byte
# . prologue
55/push-ebp
89/<- %ebp 4/r32/esp
# . save registers
51/push-ecx
# wait for an interrupt to save power on Qemu
f4/hlt
# 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 buffer-byte-addr/ecx: (addr byte)
8b/-> *Keyboard-buffer:read 1/r32/CL
81 0/subop/add %ecx Keyboard-buffer:data/imm32
# var next-key/eax: byte = *buffer-byte-addr
8a/byte-> *ecx 0/r32/AL
# if (next-key != 0) lock and remove from keyboard buffer
3d/compare-eax-with 0/imm32
{
74/jump-if-= break/disp8
fa/disable-interrupts
c6 0/subop/copy-byte *ecx 0/imm8
ff 0/subop/increment *Keyboard-buffer:read
81 4/subop/and *Keyboard-buffer:read 0x0f/imm32
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
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