Graphics Support

Quicksilva Character Board Emulation

The QS Character Board overlays the character pixel patterns at the end of the ZX81 ROM with 1K of RAM to allow both the normal and inverse characters to be redefined. Quicksilva integrated support for the QS Character Board in their games QS Scramble, QS Asteroids, QS Invaders and Cosmic Guerilla. The Chroma interface emulates the behaviour of the QS Character Board, allowing these Quicksilva games to be played in their enhanced graphical form. The QS Character Board had to be manually enabled using an onboard, and so this functionality is replicated on the Chroma interface using configuration switch 4.

Screenshots of the programs known to support the QS Character board are shown below:

Standard Display Standard Display
QS Scramble screenshot   QS Asteroids screenshot
Character Board Display Character Board Display
QS Scramble hi-res screenshot
QS Scramble (©1982 Quicksilva) QS Asteroids (©1982 Quicksilva)
Standard Display Standard Display
QS Invaders screenshot   Cosmic Guerilla screenshot
Character Board Display Character Board Display
QS Invaders hi-res screenshot
QS Invaders (©1982 Quicksilva) Cosmic Guerilla (©1983 Quicksilva)
Standard Display Standard Display
Puckman screenshot   ZX-Forth screenshot
Character Board Display Character Board Display
ZX-Forth hi-res screenshot
Puckman (©1982 Hewson Consultants) ZX-Forth (©1982 Artic Computing Ltd)

User Definable Characters

In a standard ZX81, the pixel patterns defining each character can only exist within the ROM. Two manufacturers (dk'tronics and Kayde Electronic Systems) sold a board that could to be fitted inside the ZX81 to equip it with an additional 4K of ROM containing 7 alternate character sets. In their advertising, this board was simply referred to as a Graphics ROM. Out of the 7 alternate character sets, 6 were identical between the Kayde and dk'tronics ROMs. The 4K ROM was decoded to appear immediately after the BASIC ROM in the memory map, in a region not normally populated in a ZX81 system.

The character sets provided by the ROMs are shown below:

dk'tronics Graphics ROM character set 1   dk'tronics Graphics ROM character set 2   dk'tronics Graphics ROM character set 3   dk'tronics Graphics ROM character set 4
Kayde / dk'tronics character set 1 Kayde / dk'tronics character set 2 Kayde / dk'tronics character set 3 Kayde / dk'tronics character set 4
dk'tronics Graphics ROM character set 5   Kayde Graphics ROM character set 5   dk'tronics Graphics ROM character set 6   dk'tronics Graphics ROM character set 7
dk'tronics character set 5 Kayde character set 5 Kayde / dk'tronics character set 6 Kayde / dk'tronics character set 7

The Kayde and dk'tronics boards were essentially the same and worked by extending the memory range that the display hardware could fetch character pixel patterns from. As well as containing 4K of ROM, the boards could also be fitted with 1K or 2K of RAM to allow 2 or 4 sets of user definable characters.

Kayde released software titles Space Invaders, Centipede and Peckman for use with their Graphics ROM, and dk'tronics released software titles Space Invaders, Centipede, Asteroids, Defender and Meteor Storm for use with their Graphics ROM. An example of the graphical quality achieved using a Graphics ROM is shown below:

Space Invaders (©1981 dk'tronics)

The Chroma interface provides the ability to locate the character set pixel patterns within the full range of the lower 16K region. This makes it possible to use a ROM cartridge containing a copy of the ZX81 ROM and the Kayde or dk'tronics Graphics ROM and to display the alternate character sets provided by the Graphics ROM. This allows the range of enhanced graphical games designed to utilise the ROMs to be played without the need for the original hardware.

The Chroma interface can alternatively populate the region between the BASIC ROM and the BASIC program area with 8K of RAM. This makes it possible for a user to define their own character set.


WRX True High Resolution Graphics

The WRX graphics mechanism achieves a true high resolution display using a combination of hardware and a custom video driver. It requires that the /RFSH control signal is redirected to the 16K RAM to allow pixel data to be read from it during the construction of the TV picture (on a standard ZX81 system the pixel data can only be obtained from the ROM space). The Chroma interface add the WRX graphics support to all of its onboard RAM if configuration switch 2 is set to on.

An example of the graphic quality achievable using the WRX mechanism is shown below:

StarFight (©2001 Martin Korth)

A complete description of the WRX mechanism and various video driver routines for it can be found on Wilf Rigter's website.


Chroma thumbnail