Where did the term "IRG" (as in ANTIC modes 4 & 5, multicolor text) come from?

billkendrick · October 21, 2020

Back in the late 1990s I invented "Super IRG" mode -- simply flickering two character sets to achieve more colors, inspired by APAC and ColorView modes of the era -- and used it in my game Gem Drop. The "IRG" here was meant to refer to ANTIC mode 4 [and 5] (seen in GRAPHICS 12 [and 13] on XL/XE OSes) -- the multicolor text mode. [*]

I listened to @playermissile's latest podcast that dropped today, and he mentioned Super IRG. That reminded me... I couldn't for the life of me remember where I first came across the term "IRG". Everyone just calls it "ANTIC mode 4". (People don't even seem to particularly refer to "GRAPHICS 12" -- I assume since the 400/800 OS and BASIC had no such mode.)

My own attempt to Google for relevant terms -- I'm usually thwarted by "interrecord gap" relating to cassette drive storage -- and @Savetz (via Twitter) both simultaneously reminded me of the Compute! type-in game "Thunderbird", which I plugged into my 1200XL as a kid (specifically, from Compute!'s First Book of Atari Games). The text alongside that game referred to the mode as "IRG-4", but didn't explain what that term actually meant. I suppose as a kid, I just accepted that there were 16 graphics modes, and two of them are the "IRG" ones (whatever that means). Got it! ?

In my Googling this evening, I came across other early references to the term, but frustratingly they also lacked any explanation.

Compute! #32 (Jan. 1983), "Thunderbird" [aforementioned game]
"The playing field is a mixed-mode display consisting of two rows of GRAPHICS 1 text, and 21 rows of a multicolored character mode, IRG 4."
Compute! #41 (Oct. 1983), "A Multicolor Atari Character Editor"
"two special ANTIC modes, ANTIC 4 and ANTIC 5 (or IRG 4 and 5 according to the hardware manual), allow four colors per character."
Compute! #26 (July 1982): [the earliest I could find]
"[Caverns of Mars] is constructed using a custom character set in the special four-color IRG graphics mode (a rather unexploited feature of the Atari until now)"

So now I'm super curious! ? What does "IRG" actually stand for?

Over on Twitter, @Bill Lange pointed me at the Atari Hardware Manual, and I also dug up an amusing scan of it over at Internet Archive (with LOTs of corrections and commentary from whoever owned it). The "IR" seems to mean "Instruction Register", an ANTIC-related term. As in, "Program 1 demonstrates the first of these modes, designated by Atari as Instruction Register (IR) Mode 3" (quoted from Compute! #55, Dec. 1984, "Atari's 'Hidden' Character Modes"). It really seems to be a synonym with the term "Antic mode" (as opposed to OS / BASIC "GRAPHICS mode").

So if that's the case, then does the "G" just mean "graphics", like Bill Lange suggested? ("Graphics" aka "Display", as opposed to "Blank" or "Jump" Display List instructions)

Is there actually anything special about these two multicolored text modes (ANTIC 4 & 5) that make them "IRG" modes, and all of the others not? In other words, could you say "GRAPHICS 15", aka "GRAPHICS 7½" in the pre-XL era, is just a screen full of "IRG-14" or "IRG-E"? The wording in that multicolor character editor article (Compute! #41) seems to imply it.

If so, then "whoops!" Sorry for coining the term "Super IRG". Blame the folks writing for Compute! in 1982 & 83, I guess. ?

[*] @Synthpopalooza subsequently went hog wild with the Super IRG idea (?‍♂️ ), and peeking around AtariAge forums, it looks like others have used it/them!

Edited October 21, 2020 by billkendrick
Holy crap why was it all "strike-through"!?

Rybags · October 21, 2020

Yeah, I think the term just means Instruction Register Graphics which can refer to any Antic mode but was taken on board to refer to these textual graphics modes that were not easily available at the time.

No sure if De-Re had the reference but fairly sure some of the early 80s magazines like Compute!, Byte, Analog and Antic were probably using it.

By the time the XL included it in the OS it was already in widespread use and the terms IRG and Antic 4 or 5 had already stuck.

For compatibility reasons it made sense to do your programming using a modified DList rather than a Graphics 12 call anyway, and it was only a few lines extra code and the graphics handling was fairly easy.

Graphics 15 - a different matter since Gr 8 was the only one with the same memory footprint and Gr 7 the only one with the same pixel handling. The technique there was to change the OS screen origin so you could plot to both halves of the screen.