ULA: The Spectrum's Heart

Overview

The Uncommitted Logic Array (ULA) was Sinclair’s masterstroke of cost engineering. Instead of multiple discrete chips, a single custom IC handled video generation, memory contention, keyboard matrix scanning, tape I/O, and the border colour. This integration made the £125 Spectrum possible.

Fast facts

Manufacturer: Ferranti (later versions by other fabs).
Function: display, memory arbitration, I/O.
Video output: PAL composite via RF modulator.
Colour depth: 8 colours × 2 brightness levels = 15 unique colours.

Display generation

The ULA reads display memory and generates the video signal:

Memory region	Size	Purpose
$4000-$57FF	6KB	Pixel data (256×192, 1bpp)
$5800-$5AFF	768B	Attribute data (32×24 cells)

Screen layout quirks

The pixel data isn’t linearly arranged—it’s interleaved by character rows:

Address = $4000 + ((y & 0xC0) << 5) + ((y & 0x07) << 8) + ((y & 0x38) << 2) + (x >> 3)

This unusual layout made scrolling challenging but matched the ULA’s line-drawing requirements.

Attribute format

Each 8×8 cell has one attribute byte:

Bits	Purpose
0-2	INK (foreground) colour
3-5	PAPER (background) colour
6	BRIGHT (affects both ink and paper)
7	FLASH (swap ink/paper every 16 frames)

Colour clash

The one-attribute-per-cell design created the Spectrum’s infamous colour clash:

Each 8×8 pixel block can only have two colours.
Multicoloured sprites crossing cell boundaries show jarring colour changes.
Games designed around this—or embraced the monochrome look.

Memory contention

The ULA and CPU share the same RAM:

During active display, the ULA takes priority.
CPU access to $4000-$7FFF is delayed during certain T-states.
Contention follows a predictable pattern per scanline.
Demo coders exploit this for precise timing.

Contention pattern (per 8 T-states)

T-state: 0  1  2  3  4  5  6  7
Delay:   6  5  4  3  2  1  0  0

I/O ports

Port	Read	Write
$FE	Keyboard + ear input	Border colour + mic/ear
$FF	Floating bus (video data)	—

Keyboard scanning

Half-row selection via address lines:

A8-A15	Keys read at $FE
$FEFE	SHIFT, Z, X, C, V
$FDFE	A, S, D, F, G
$FBFE	Q, W, E, R, T
$F7FE	1, 2, 3, 4, 5
$EFFE	0, 9, 8, 7, 6
$DFFE	P, O, I, U, Y
$BFFE	ENTER, L, K, J, H
$7FFE	SPACE, SYM, M, N, B

Border colour

Writing to port $FE sets the border:

ld a, 2          ; red
out ($fe), a     ; set border colour

Bits 0-2 select the colour (0-7). Games used border stripes for loading screens and effects.

Cultural impact

The ULA’s compromises—colour clash, contention, limited colours—became the Spectrum’s identity. Programmers learnt to work with these constraints, creating a distinct visual style. The techniques developed here still inform retro game design.

One chip to rule them all