• Please review our updated Terms and Rules here

Model III/4 Display Question

deathshadow

Veteran Member
Joined
Jan 4, 2011
Messages
1,378
Playing around on my 4P in model III mode, and I could swear I'm remembering this different. Was the model III character set that radically different from the Model 1?

I could have sworn that apart from upper/lower case they were identical, but it looks like characters 0..0x20 and 0xC0 to 0xFF are different. Researching it doesn't line up with this old sod's memory. Though I went through so many third party character ROM on the model 1 that could be messing with my memory. Hell I had a lower case kit that did proper descenders, a full whopping 2 pixels instead of one.

Is it correct that the model 1 had an entirely different set of characters in the bottom 32, and repeated the semigraphics normally at 0x80 to 0xBF in the top 64? Unlike the model III that has a whole slew of (semi-useless for English language users) characters up top?

And resolution. I could have sworn they were all 6 pixels wide per character, with the font stored sideways, but the model III manual and visual inspection says they're 8 pixels wide, not six. Could have sworn the model 1 and 3 were both 384x192... was the III really 512x192? (and therefor 640x288 for model 4?)

That change in the bottom 32 should have led to some software not working right on the III... is that correct too?

I think I'm just having a brainfart. It's been too many years since I dealt with these platforms. Which is sad. As much as I learned on the ELF, and the ZX-80 has a warm spot in my heart, the model 1 was the first platform I actually felt like I was "accomplishing something."
 
And resolution. I could have sworn they were all 6 pixels wide per character, with the font stored sideways

I’m not sure what you mean here by “the font stored sideways”, both machines have perfectly conventional character generators; the Model I‘s only has 5 output lines because it was designed to store 5x7 character matrixes and the address lines are split into two groups for “character” and “row” so the package doesn‘t look like a conventional ROM, but it effectively behaves like one.

And yes, the Model III increases the size of the character cells to 8x12 instead of 6x12 for a total pixel display of 512x192 instead of 384x192. Interesting factoid, though: both machines have pixel clocks around 10mhz; the Model I just has a ton more horizontal border. Which befits it needing to use an only very lightly modified TV set instead of a dedicated monitor.

And, as noted, the Model III has all those graphics/kanji characters in the top 64 positions while the Model I just has two copies of the graphics blocks. On both machines the graphics blocks are not stored in the character ROM, they’re generated by a little handful of logic sitting alongside it. The Model III‘s character ROM is twice as big as the I’s; a I (with lowercase modification) has access to 128 different glyphs for memory values 0-127, the III has that 128 plus the two selectable sets of 64 from 192-255.

Finally, note that because of all the variations in Model I character ROMs and the oddities imposed by third party lowercase mods it is totally inconsistent what characters you get for 0-31. The “official” Radio Shack mod ROM eliminated the graphics characters hidden down there and replaced them with a second set of uppercase letters so they could improve compatibility with broken uppercase only software without a switch or weird memory-mangle tricks.
 
That change in the bottom 32 should have led to some software not working right on the III... is that correct too?

This deserves a little explanation: Without a lowercase mod only 64 characters of the 128 in the ROM were available because the video RAM was only 7 bits wide; effectively the machine can only display unique characters for memory values 32 through 95. Why they decided this ridonkulous hackery to shuffle the ascii codes around was better than paying for one more SRAM chip… well, I guess it’s obvious, it was slightly cheaper and the $4 they saved was *totally* worth doing without lowercase letters, right?

Anyway, a critical thing about this is there are a few places in the Level II ROM where it writes a 1 through 26 to vram instead of the proper 65-90 when outputting letters. (because that mangling they did to split their 7 bits between characters and graphics meant writing either value had the same result.) This is why early lowercase mods that exposed those extra bonus graphics characters at 0-31 *had* to have a driver fix loaded and usually had a disable switch. (They also sometimes retained hackery to automatically translate writes to VRAM from 0-31 to 32-95, which also prevented you from ever seeing those bonus characters at all.) So… long and short of it, there’s very little software for the Model I that ever used them.

I do think there is some software for the I besides the ROM that makes that mistake with writing the wrong codes for uppercase characters, I think that does break it on a III if it’s not patched.
 
Hey deathshadow,

Saw your Elephant Memory Systems avatar, and it reminded me that just yesterday I was thinking about their 5-1/4" floppies and just how utterly terrible they were.

My god were they awful, lol. Great logo, though!
 
@Eudimorphodon thanks for the info. It would seem that I never used an unmodified model 1 long enough to encounter the limitations of the native character ROM implementation.

I think I must have gotten the lower case ROM the same time I got the Level 2 basic and 16k upgrade... because whilst I know I started with a 4k Level 1 model, it was pretty damned useless. Still a step up from the ELF...
 
Back
Top