Started implementing PS/2 Keyboard driver

pat80-computer/software/z80-assembly/os/drivers/keyboard.asm

@@ -1,4 +1,5 @@
 ; Keyboard driver
+; Direct keyboard grid control (direct keys addressing, without keyboard controller)
 ; @author Daniele Verducci
 ;
 ; Requires declaration of following pointers, one for every column of the keys grid:

pat80-computer/software/z80-assembly/os/drivers/ps2_keyboard.asm

@@ -0,0 +1,51 @@
+; PS/2 Keyboard driver
+;
+; Based on PS/2 protocol as documented on http://www.lucadavidian.com/2017/11/15/interfacing-ps2-keyboard-to-a-microcontroller/
+;
+; The CLK and Data pin of the PS/2 keyboard are fed into two cascated serial-in parallel-out shift registers.
+; Their outputs are connected to the Pat80 data bus via a buffer activated by the selected 
+; I/O EN signal and their RESET is connected to I/O Address line 0 of the keyboard I/O port. 
+; Being RESET active low, they will be erased when the PAT80 reads (or writes) anything at address 0 
+; of the keyboard I/0 port.
+;
+; Thus, the read cycle is:
+; - Read address 1 of the I/O port (the data bus will contain read keycode)
+; - Read address 0 of the I/O port (the shift registers will be reset)
+; The read keycode must be interpreted based on PS/2 Scan Codeset 2
+;
+; NOTE: The keyboard controller circuit throws away the MSB (uses only the lower 7 bits), because this allows
+; for using a single buffer chip instead of two (the freed up line is used by the PAT80 to reset the shift
+; registers). This means that the few keys with keycodes > 0x0F are not readable and that the break code is
+; seen by PAT80 not as 0xF0, but 0x70. This also means that the 0 of numeric keypad on extended keyboards will
+; behave strangely (will drop next pressed key). This is not a problem, as the computer, once completed, will
+; have a 60% keyboard, without any of the unusable keys.
+
+include "ps2_keyboard_scancodeset2.asm"    ; PS/2 Scan Codeset 2 mappings
+
+; config (IO port 1)
+PS2KEYB_CLEAR_REG: EQU IO_1
+PS2KEYB_DATA_REG: EQU IO_1 + 1
+
+PS2KEYB_BREAK: EQU 0xF0 - %10000000    ; The MSB is dropped: see NOTE on intro above
+
+; Reads a single character. 0s are ignored (can be used with keyboard).
+; Doesn't check DATA_AVAILABLE register of parallel port, because a 0 byte
+; is ignored anyway (it represents the ASCII NUL control char).
+; @return A The read character
+PS2Keyb_readc:
+    in a, (PS2KEYB_DATA_REG)    ; reads a character
+    add a, 0
+    jp z, Term_readc     ; if char is 0 (NULL), user didn't press any key: wait for character
+	; check if code is a Break Code (0xF0). If it is, discard next key as it is a released key
+	ld b, a    ; save a
+	cp 0xF0    ; compare a with Break Code
+	jp z, ps2keyb_readc_discard
+	; we read a valid character: clean key registers
+	in a, PS2KEYB_CLEAR_REG
+	; TODO: Interpretare keycode con lo scan code set
+	;ld a, b    ; restore a
+    ret ; returns in the a register
+	ps2keyb_readc_discard:
+		; waits for next non-0 keycode and discards it
+		; TODO
+		jp PS2Keyb_readc    ; go back and wait for another keycode