Keyboard Reading

Overview

Read keyboard input by polling the I/O ports directly. The Spectrum’s keyboard is arranged in a matrix of 8 rows, each accessible via a specific port address. Keys are active low (0 = pressed). Use when you need responsive input without relying on the ROM.

Code

; =============================================================================
; KEYBOARD READING - ZX SPECTRUM
; Read keyboard matrix via I/O ports
; Taught: Game 1 (Ink War), Unit 1
; CPU: ~50 cycles | Memory: ~40 bytes
; =============================================================================

KEY_PORT    equ     $fe

; Keyboard row addresses (active low)
ROW_QAOP    equ     $fb             ; Q W E R T (bits: T R E W Q)
ROW_ASDF    equ     $fd             ; A S D F G (bits: G F D S A)
ROW_YUIOP   equ     $df             ; Y U I O P (bits: P O I U Y)
ROW_12345   equ     $f7             ; 1 2 3 4 5 (bits: 5 4 3 2 1)
ROW_09876   equ     $ef             ; 0 9 8 7 6 (bits: 6 7 8 9 0)
ROW_SPACE   equ     $7f             ; Space, Sym, M, N, B

; Read keyboard and return direction code
; Returns: A = 0 (none), 1 (up), 2 (down), 3 (left), 4 (right)
read_keyboard:
    xor     a
    ld      (key_pressed), a    ; Clear previous

    ; Check Q (up)
    ld      a, ROW_QAOP
    in      a, (KEY_PORT)
    bit     0, a                ; Q is bit 0
    jr      nz, .not_q
    ld      a, 1
    ld      (key_pressed), a
    ret
.not_q:
    ; Check A (down)
    ld      a, ROW_ASDF
    in      a, (KEY_PORT)
    bit     0, a                ; A is bit 0
    jr      nz, .not_a
    ld      a, 2
    ld      (key_pressed), a
    ret
.not_a:
    ; Check O (left)
    ld      a, ROW_YUIOP
    in      a, (KEY_PORT)
    bit     1, a                ; O is bit 1
    jr      nz, .not_o
    ld      a, 3
    ld      (key_pressed), a
    ret
.not_o:
    ; Check P (right)
    ld      a, ROW_YUIOP
    in      a, (KEY_PORT)
    bit     0, a                ; P is bit 0
    jr      nz, .not_p
    ld      a, 4
    ld      (key_pressed), a
.not_p:
    ret

key_pressed:
    defb    0

Usage:

main_loop:
    halt                        ; Wait for frame
    call    read_keyboard
    ld      a, (key_pressed)
    or      a
    jr      z, main_loop        ; No key pressed

    cp      1
    jr      z, handle_up
    cp      2
    jr      z, handle_down
    ; ... etc
    jr      main_loop

Trade-offs

Aspect	Cost
CPU	~50 cycles per read
Memory	~40 bytes
Limitation	No debouncing (keys may auto-repeat)

When to use: Any game needing keyboard input. ROM-free approach works on all Spectrums.

When to avoid: When you need debounced input for menus or single-press detection.

Keyboard Matrix Reference

Port	Bit 0	Bit 1	Bit 2	Bit 3	Bit 4
$F7	1	2	3	4	5
$EF	0	9	8	7	6
$FB	Q	W	E	R	T
$FD	A	S	D	F	G
$DF	P	O	I	U	Y
$BF	Enter	L	K	J	H
$7F	Space	Sym	M	N	B
$FE	Shift	Z	X	C	V

Related

Patterns: Game Loop (HALT)

Vault: ZX Spectrum