ZX Spectrum Hardware Ports

The ZX Spectrum uses I/O ports to communicate with hardware devices. This reference covers all standard Spectrum ports with focus on the multi-purpose port $FE.

Port $FE - The Multi-Purpose Port

Port $FE is the most important port on the Spectrum. It controls border colour, reads the keyboard, controls the speaker, and handles tape I/O.

Port $FE Output (Writing)

Usage: OUT ($FE),A

Bit Layout:

Bit	7	6	5	4	3	2	1	0
Purpose	—	—	—	Speaker	Tape	Border	Border	Border
Value	x	x	x	MIC	EAR	Border	Border	Border

Bit Functions:

Bits 0-2: Border colour (0-7)
Bit 3: Tape output (EAR socket)
Bit 4: Speaker control (0 = off, 1 = on)
Bits 5-7: Not used (but should be preserved)

Example - Change Border:

LD A,2           ; Red border
OUT ($FE),A

Example - Make Sound:

; Toggle speaker bit for simple beep
LD A,$10         ; Speaker on, black border
OUT ($FE),A
; Delay
LD A,$00         ; Speaker off
OUT ($FE),A

Example - Preserve Upper Bits:

; Read current value, modify border only
IN A,($FE)       ; Read current port state
AND $F8          ; Clear border bits (0-2)
OR 5             ; Set to cyan (5)
OUT ($FE),A      ; Write back

Port $FE Input (Reading)

Usage: IN A,($FE)

Bit Layout:

Bit	7	6	5	4	3	2	1	0
Purpose	—	Tape	—	—	Key	Key	Key	Key
Active	x	EAR	x	x	Row bit 4	Row bit 3	Row bit 2	Row bit 1

Bit Functions:

Bits 0-4: Keyboard row data (active low - 0 = pressed)
Bit 5: Not used
Bit 6: Tape input (EAR socket)
Bit 7: Not used

Important: Keyboard reading requires setting the upper byte of BC to select keyboard half-row.

Keyboard Reading

The Spectrum keyboard is arranged in a matrix of 8 half-rows × 5 keys each.

Keyboard Matrix

To read keyboard:

Set BC register: High byte selects half-row, low byte = $FE
Read with IN A,(C)
Test bits 0-4 (active low: 0 = pressed, 1 = not pressed)

Keyboard Half-Row Map

BC High	Half-row	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
$FE	Row 0	V	C	X	Z	SHIFT
$FD	Row 1	G	F	D	S	A
$FB	Row 2	T	R	E	W	Q
$F7	Row 3	5	4	3	2	1
$EF	Row 4	6	7	8	9	0
$DF	Row 5	Y	U	I	O	P
$BF	Row 6	H	J	K	L	ENTER
$7F	Row 7	B	N	M	SYM	SPACE

Reading Single Key

Example - Read Q key:

        LD BC,$FBFE       ; Half-row $FB (Q-T row)
        IN A,(C)          ; Read keyboard
        BIT 0,A           ; Test bit 0 (Q key)
        JR Z,QPressed     ; Jump if Q pressed (bit = 0)

Example - Read Arrow Keys (using QAOP):

; Read Q (up) key
        LD BC,$FBFE       ; Q-T row
        IN A,(C)
        BIT 0,A
        JR Z,MoveUp

; Read A (down) key
        LD BC,$FDFE       ; A-G row
        IN A,(C)
        BIT 0,A
        JR Z,MoveDown

; Read O (right) key
        LD BC,$DFFE       ; P-Y row
        IN A,(C)
        BIT 1,A
        JR Z,MoveRight

; Read P (left) key
        LD BC,$DFFE       ; Same row
        IN A,(C)
        BIT 0,A
        JR Z,MoveLeft

Reading Multiple Keys Simultaneously

The Spectrum supports multiple simultaneous key presses (within matrix limitations).

Example - Detect multiple keys:

; Read Q-T row
        LD BC,$FBFE
        IN A,(C)
        LD D,A            ; Save for later

        BIT 0,D           ; Q pressed?
        CALL Z,HandleQ

        BIT 1,D           ; W pressed?
        CALL Z,HandleW

Scan All Keys

Example - Detect any key press:

ScanKeyboard:
        LD BC,$FEFE       ; Start with row 0
        LD D,8            ; 8 rows to scan
ScanLoop:
        IN A,(C)          ; Read row
        AND $1F           ; Mask to key bits
        CP $1F            ; All bits set = no keys
        JR NZ,KeyFound    ; Jump if any key pressed

        RLC B             ; Next row (rotate high byte)
        DEC D
        JR NZ,ScanLoop

        ; No keys found
        RET

KeyFound:
        ; Key detected - A contains key bits
        RET

Ghost Keys and Limitations

Matrix ghosting: Pressing certain 3-key combinations can register phantom 4th key.

Example problem:

Press: Q + A + O
Ghost: P may appear pressed

Solution: Use diagonally opposite keys in your control schemes (e.g., QAOP works well).

Speaker/Beeper ($FE Bit 4)

The Spectrum has a simple 1-bit speaker controlled by port $FE bit 4.

Simple Beep

Beep:
        LD B,200          ; Duration counter
BeepLoop:
        LD A,$10          ; Speaker on
        OUT ($FE),A

        LD E,50           ; Frequency delay
Delay1: DEC E
        JR NZ,Delay1

        LD A,$00          ; Speaker off
        OUT ($FE),A

        LD E,50
Delay2: DEC E
        JR NZ,Delay2

        DJNZ BeepLoop
        RET

Variable Pitch Beep

; HL = pitch (higher = higher pitch)
; B = duration
ToneBeep:
ToneLoop:
        LD A,$10
        OUT ($FE),A
        PUSH HL
ToneDelay1:
        DEC HL
        LD A,H
        OR L
        JR NZ,ToneDelay1
        POP HL

        LD A,$00
        OUT ($FE),A
        PUSH HL
ToneDelay2:
        DEC HL
        LD A,H
        OR L
        JR NZ,ToneDelay2
        POP HL

        DJNZ ToneLoop
        RET

Preserving Border Colour with Speaker

Problem: Writing to port $FE affects border and speaker.

Solution: Read current value, modify only speaker bit:

        IN A,($FE)        ; Read current port
        AND $F8           ; Clear border bits
        LD C,A            ; Save base value

        ; Speaker ON
        LD A,C
        OR $10            ; Set speaker bit
        OUT ($FE),A

        ; Delay...

        ; Speaker OFF
        LD A,C
        OUT ($FE),A       ; Restore (speaker bit = 0)

ULA Port ($FE) - Complete Specification

Output Bit Summary

Bit	Function	Effect
0	Border LSB	Border colour bit 0
1	Border	Border colour bit 1
2	Border MSB	Border colour bit 2
3	MIC	Tape output (cassette save)
4	Speaker	1-bit speaker output
5-7	Unused	No effect (recommended: write 0)

Border colours (bits 0-2):

%000 = 0 = Black      %100 = 4 = Green
%001 = 1 = Blue       %101 = 5 = Cyan
%010 = 2 = Red        %110 = 6 = Yellow
%011 = 3 = Magenta    %111 = 7 = White

Input Bit Summary

Bit	Function	Effect
0-4	Keyboard	5 keys per half-row (active low)
5	Unused	Always reads 1
6	EAR	Tape input signal
7	Unused	Always reads 1

Other Spectrum Ports

Port $7FFD - 128K Memory Paging (128K models only)

Not available on 48K Spectrum. Used for bank switching on 128K models.

Port $FFFD - AY-3-8912 Sound Chip (128K models only)

3-channel sound chip on 128K/+2/+3 models. Not present on 48K.

Kempston Joystick - Port $1F

Not a standard Spectrum feature - third-party add-on.

Read: IN A,($1F)

Bit	Function
0	Right
1	Left
2	Down
3	Up
4	Fire

Complete I/O port reference