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The ULAplus specification describes an enhanced ULA for the ZX Spectrum. It can be implemented as a plug-in replacement for the ULA, in emulators, or in modern hardware such as the ZX Spectrum Vega. It is designed for maximum compatibility with existing software. This information supersedes all existing versions of the specification, including those with later version numbers.

The final official version of the specification is version 1.1.

Revision History

Version 1.1

Revised for the release of ZXDS 1.3 incorporating changes from the OpenCores ULAplus implementation. This version rationalizes the specification. Software written for the standard mode of the new specification will run unmodified on earlier implementations.

  • Includes the Timex video modes (optional)
  • Changes the preferred way of multiplexing the 2-bit blue value to obtain a 3-bit blue value.
  • Adds 256 grayscale support (optional)
  • Deprecates HSL and CMYK support

Version 1.0

First published version of the specification.

I/O ports

ULAplus is controlled by two ports.

0xBF3B is the register port (write only)

The byte output will be interpreted as follows:

Bits 0-5: Select the register sub-group
Bits 6-7: Select the register group. Two groups are currently available:

     00 - palette group

          When this group is selected, the sub-group determines the
          entry in the palette table (0-63).

     01 - mode group

          The sub-group is (optionally) used to mirror the video functionality of Timex port #FF as follows:

          Bits 0-2: Screen mode. 000=screen 0, 001=screen 1, 010=hi-colour, 110=hi-res (bank 5)
                                 100=screen 0, 101=screen 1, 011=hi-colour, 111=hi-res (bank 7)
          Bits 3-5: Sets the screen colour in hi-res mode.
              000 - Black on White     100 - Green on Magenta
              001 - Blue on Yellow     101 - Cyan on Red
              010 - Red on Cyan        110 - Yellow on Blue
              011 - Magenta on Green   111 - White on Black

0xFF3B is the data port (read/write)

When the palette group is selected, the byte written will describe the color.

When the mode group is selected, the byte output will be interpreted as follows:

Bit 0: ULAplus palette on (1) / off (0)
    1: (optional) grayscale: on (1) / off (0) (same as turing the color off on the television)

Implementations that support the Timex video modes use the #FF register as the primary means to set the video mode, as per the Timex machines. It is left to the individual implementations to determine if reading the port returns the previous write or the floating bus.

GRB palette entries

G3R3B2 encoding

For a device using the GRB colour space the palette entry is interpreted as follows

Bits 0-1: Blue intensity.
Bits 2-4: Red intensity.
Bits 5-7: Green intensity.

This colour space uses a sub-set of 9-bit GRB. The missing lowest blue bit is set to OR of the other two blue bits (Bb becomes 000 for 00, and Bb1 for anything else). This gives access to a fixed half the potential 512 colour palette. The reduces the jump in intensity in the lower range in the earlier version of the specification. It also means the standard palette can now be represented by the ULAplus palette.

Grayscale palette entries

In grayscale mode, each palette entry describes an intensity from zero to 255. This can be achieved by simply removing the colour from the output signal.


Although in theory 64 colours can be displayed at once, in practice this is usually not possible except when displaying colour bars, because the four CLUTs are mutually exclusive; it is not possible to mix colours from two CLUTs in the same cell. However, with software palette cycling it is possible to display all 256 colours on screen at once.


The 64 colour mode lookup table is organized as 4 palettes of 16 colours.

Bits 7 and 6 of each Spectrum attribute byte (normally used for FLASH and BRIGHT) will be used as an index value (0-3) to select one of the four colour palettes.

Each colour palette has 16 entries (8 for INK, 8 for PAPER). Bits 0 to 2 (INK) and 3 to 5 (PAPER) of the attribute byte will be used as indexes to retrieve colour data from the selected palette.

With the standard Spectrum display, the BORDER colour is the same as the PAPER colour in the first CLUT. For example BORDER 0 would set the border to the same colour as PAPER 0 (with the BRIGHT and FLASH bits not set).

The complete index can be calculated as

ink_colour = (FLASH * 2 + BRIGHT) * 16 + INK
paper_colour = (FLASH * 2 + BRIGHT) * 16 + PAPER + 8

When scaling 3-bits of colour data to more bits for emulators that operate in high colour mode, simply concatenate the bits repeatedly and then truncate to as many bits as needed. For example, for 8-bits the following conversion should be used:


where h is the high bit, m is the middle bit, and l is the low bit of the original 3-bit value.

Extension to the ZX-State (SZX) format

This is now documented on the ZX-State format page itself.

Extension to the SCR format

A 6912 byte .SCR file contains a standard Spectrum screen.

A 6976 byte .SCR file contains a standard Spectrum screen followed by 64 colour registers.

A 12288 byte .SCR file contains a Timex hi-colour screen.

A 12352 byte .SCR file contains a Timex hi-colour screen followed by 64 colour registers.

A 12289 byte .SCR file contains a Timex hi-res screen.

A 12353 byte .SCR file contains a Timex hi-res screen followed by the hi-res colour information that was dumped from port 255, followed by 64 colour registers.

Palette file format

The palette format doubles as the BASIC patch loader. This enables you to edit patches produced by other people.

; 64 colour palette file format (internal) - version 1.0
; copyright (c) 2009 Andrew Owen
; The palette file is stored as a BASIC program with embedded machine code


db 0x00 ; program file
db 0x14, 0x01, "64colour" ; file name
dw 0x0097 ; data length
dw 0x0000 ; autostart line
dw 0x0097 ; program length


; 3635"+VAL "256"*PEEK VAL "23636"
; )+VAL "48"): LOAD "": REM

db 0x00, 0x00, 0x93, 0x00, 0xf9, 0xc0, 0x28, 0x28
db 0xbe, 0xb0, 0x22, 0x32, 0x33, 0x36, 0x33, 0x35
db 0x22, 0x2b, 0xb0, 0x22, 0x32, 0x35, 0x36, 0x22
db 0x2a, 0xbe, 0xb0, 0x22, 0x32, 0x33, 0x36, 0x33
db 0x36, 0x22, 0x29, 0x2b, 0xb0, 0x22, 0x34, 0x38
db 0x22, 0x29, 0x3a, 0xef, 0x22, 0x22, 0x3a, 0xea


di ; disable interrupts
ld hl, 38 ; HL = length of code
add hl, bc ; BC = entry point (start) from BASIC
ld bc, 0xbf3b ; register select
ld a, 64 ; mode group
out (c), a ;
ld a, 1 ;
ld b, 0xff ; choose register port
out (c), a ; turn palette mode on
xor a ; first register


ld b, 0xbf ; choose register port
out (c), a ; select register
ex af, af' ; save current register select
ld a, (hl) ; get data
ld b, 0xff ; choose data port
out (c), a ; set it
ex af, af' ; restore current register
inc hl ; advance pointer
inc a ; increase register
cp 64 ; are we nearly there yet?
jr nz, setreg ; repeat until all 64 have been done
ei ; enable interrupts
ret ; return

; this is where the actual data is stored. The following is an example palette.


db 0x00, 0x02, 0x18, 0x1b, 0xc0, 0xc3, 0xd8, 0xdb ; INK
db 0x00, 0x02, 0x18, 0x1b, 0xc0, 0xc3, 0xd8, 0xdb ; PAPER
db 0x00, 0x03, 0x1c, 0x1f, 0xe0, 0xe3, 0xfc, 0xff ; +BRIGHT
db 0x00, 0x03, 0x1c, 0x1f, 0xe0, 0xe3, 0xfc, 0xff ;
db 0xdb, 0xd8, 0xc3, 0xc0, 0x1b, 0x18, 0x02, 0x00 ; +FLASH
db 0xdb, 0xd8, 0xc3, 0xc0, 0x1b, 0x18, 0x02, 0x00 ;
db 0xff, 0xfc, 0xe3, 0xe0, 0x1f, 0x1c, 0x03, 0x00 ; +BRIGHT/
db 0xff, 0xfc, 0xe3, 0xe0, 0x1f, 0x1c, 0x03, 0x00 ; +FLASH


db 0x0d 


A rapidly growing collection of software is available including a palette editor, palette files to re-colour existing games, a 6-bit RGB colour extension to CP/M plus, a slideshow of converted Commodore 64 hi-res pictures, and brand new games, all of which can be found at [http://sites.google.com/site/ulaplus/home].

The palette editor enables you to create a palette for use with existing software without needing to rewrite the software. You can save the palette and load it before loading the original software as normal. In this way all existing software can be re-coloured without any programming knowledge required.

An introduction to using the ULAplus palette from BASIC is also available [1]

Article license information

This article uses material from the "ZX Spectrum 64 Colour Mode" article on the ZX Spectrum technical information wiki at Wikia and is released under the Creative Commons Attribution-Share Alike License.