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I wrote yesterday that the ZX81 has a native graphics resolution of 64x48. That's not strictly true. It has a pixel resolution of 256x192, but has a character map of 32x24. The PLOT routines use the graphics block characters as pseudo-pixels to effect a resolution of 64x48.

L Break Into Program

@breakintoprogram.co.uk

9 Sept

To confuse matters more, there are a handful of techniques to force it to do hires graphics, and there are a handful of games that do just that, including an incredible port of Manic Miner.

youtube.comManic Miner on the ZX81 in High Resolution - Unbelievable but trueYouTube video by Villordsutch

L Break Into Program

@breakintoprogram.co.uk

9 Sept

Oh and the character map, also known as D_FILE. You'd think that this is a 32x24 array in a static position in the memory map. Ha! Gotcha!!

zx81keyboardadventure.comPeeking into the ZX81s Screen MazeRetro computer blogging on games, software, electronics, Arduino, Raspberry PI & projects for the Sinclair ZX81, ZX Spectrum & retro other computers.

L Break Into Program

@breakintoprogram.co.uk

9 Sept

And how it generates the video is also incredibly bonkers and very Sinclair.

blog.tynemouthsoftware.co.ukHow the ZX81 Generates VideoNow the Minstrel kits are back in stock, it's time for the second in this series of posts from my Patreon, this one covering the video outpu...

L Break Into Program

@breakintoprogram.co.uk

9 Sept

Why did they go to such lengths? For one, it inherited it from the ZX80. Secondly, you can shrink the D_FILE for systems with less than 4K; each line is effectively a newline terminated string (byte value 118). So an empty display takes up just 25 bytes, not 768. It expands each line as required.

L Break Into Program

@breakintoprogram.co.uk

The display is partly CPU generated; the ZX80/81 NEWLINE character (118) is not ASCII 13 or 10 as you'd expect because 118 is the Z80 opcode HALT. In essence the ULA fools the CPU into thinking it is reading NOP instructions whilst it generates the video until it hits a HALT instruction at the EOL.

9/9/26, 10:22 am0 reposts3 likes

L Break Into Program

@breakintoprogram.co.uk

9 Sept

Anyway, it's a lot more complicated than that. Read the linked Tynemouth Software blog post on it for more details. It's just one of those "Ah yes, I understand that is why that is" things. With a little more money they could have done all the video generation in ULA.

Richard Carlsson

@richcarl.bsky.social

9 Sept

Always wondered what they did in the ZX80. Now that I know, I must say it's the most bonkers solution I've ever seen, all in good Sinclair style. Hijack both the instruction fetch *and* the refresh, why not?

L Break Into Program

@breakintoprogram.co.uk

9 Sept

Which they did for the Spectrum. No more SLOW mode, it was always running in FAST. Well, apart from lower 16K RAM contention. But that's a tale for another thread.