52 lines
2.5 KiB
NASM
52 lines
2.5 KiB
NASM
; PS/2 Keyboard driver
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;
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; Based on PS/2 protocol as documented on http://www.lucadavidian.com/2017/11/15/interfacing-ps2-keyboard-to-a-microcontroller/
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;
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; The CLK and Data pin of the PS/2 keyboard are fed into two cascated serial-in parallel-out shift registers.
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; Their outputs are connected to the Pat80 data bus via a buffer activated by the selected
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; I/O EN signal and their RESET is connected to I/O Address line 0 of the keyboard I/O port.
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; Being RESET active low, they will be erased when the PAT80 reads (or writes) anything at address 0
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; of the keyboard I/0 port.
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;
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; Thus, the read cycle is:
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; - Read address 1 of the I/O port (the data bus will contain read keycode)
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; - Read address 0 of the I/O port (the shift registers will be reset)
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; The read keycode must be interpreted based on PS/2 Scan Codeset 2
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;
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; NOTE: The keyboard controller circuit throws away the MSB (uses only the lower 7 bits), because this allows
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; for using a single buffer chip instead of two (the freed up line is used by the PAT80 to reset the shift
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; registers). This means that the few keys with keycodes > 0x0F are not readable and that the break code is
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; seen by PAT80 not as 0xF0, but 0x70. This also means that the 0 of numeric keypad on extended keyboards will
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; behave strangely (will drop next pressed key). This is not a problem, as the computer, once completed, will
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; have a 60% keyboard, without any of the unusable keys.
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include "ps2_keyboard_scancodeset2.asm" ; PS/2 Scan Codeset 2 mappings
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; config (IO port 1)
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PS2KEYB_CLEAR_REG: EQU IO_1
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PS2KEYB_DATA_REG: EQU IO_1 + 1
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PS2KEYB_BREAK: EQU 0xF0 - %10000000 ; The MSB is dropped: see NOTE on intro above
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; Reads a single character. 0s are ignored (can be used with keyboard).
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; Doesn't check DATA_AVAILABLE register of parallel port, because a 0 byte
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; is ignored anyway (it represents the ASCII NUL control char).
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; @return A The read character
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PS2Keyb_readc:
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in a, (PS2KEYB_DATA_REG) ; reads a character
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add a, 0
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jp z, Term_readc ; if char is 0 (NULL), user didn't press any key: wait for character
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; check if code is a Break Code (0xF0). If it is, discard next key as it is a released key
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ld b, a ; save a
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cp 0xF0 ; compare a with Break Code
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jp z, ps2keyb_readc_discard
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; we read a valid character: clean key registers
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in a, PS2KEYB_CLEAR_REG
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; TODO: Interpretare keycode con lo scan code set
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;ld a, b ; restore a
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ret ; returns in the a register
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ps2keyb_readc_discard:
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; waits for next non-0 keycode and discards it
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; TODO
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jp PS2Keyb_readc ; go back and wait for another keycode
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