C64 is no comparison - the C64 just has so insanely much more memory to make the Inty essentially a pocket calculator in comparison. (Literally, I have a late '70s pocket calculator that has more RAM than the base Intellivision. HP 41C with memory expansion - gives it about the same amount of memory as an Intellivision plus ECS.) That was the biggest problem with the Intellivision - while the CPU was "decent" (2 MHz, 16-bit CPU,) that should have been almost on par with the CPU in the original IBM PC. But it was severely hobbled in RAM. The graphics unit was also very limited, being much lower resolution than the Coleco and C64, as well as having fewer available colors. 159x96 resolution with 16 colors. That's it.
In theory, the Intellivision can address 64 KB, though (combined RAM+ROM.) If someone were to make a game that *NEEDED* more memory, they could put RAM on the cartridge. (UCSF Chess did this! Adding 2 KB of RAM to add enough RAM for the 'vs. computer' mode to be possible.) So if you were to program a game that fit in a tiny amount of ROM (8 KB, say,) but needed a large amount of RAM, you could add ~48 KB of RAM to the cartridge, That would allow for games nearly as complex as a C64 - as long as your code+resources fit in a small area (or you implement bank-switching to allow more code to be loaded.)
But you can't get around the resolution and color issue. You can "hack" the color by changing between two colors very fast to "emulate" another color (flash between red and blue really fast to make it look purple,) and there are games that have done this (Tower of Doom, for one.) But it is CPU intensive, so to my knowledge, has only been used on fairly static "intro" or "transition" scenes, not in-game to add more colors. And the resolution is a hard limit. ColecoVision had higher 256×192 resolution, but fewer sprites, and while it could display the same number of colors, the way it could apply them was more limited. In the end, programmers for Coleco were able to make use of the much higher resolution to better effect, in spite of its other limits.
Some of the points in your comparison are not really fair, and some of the limitations you described apply only to the original Mattel games, and were mostly due to cost constraints of cartridge production of the time.
I believe the Intellivision CPU by itself can address up to 1Mb of memory. The Intellivision hardware puts some constraints on this, tough. Current home-brews have an available ROM address space of 42K (that's Kilo-Words, not Kilo-Bytes -- for the CPU addresses 16-bit words), strewn about 5 non-contiguous segments -- and that's before adding the ability to bank-switch, which gives you ~15 overlaid banks per segment.
Modern cartridge PCBs include just about 8K of additional RAM (again, that's 16-bit Kilo-Words, not Kilo-Bytes, since the CPU addresses 16-bit memory), which is plenty for long, complex, and highly intricate games. These are features that could have been used back in the day, but hardware costs (specifically for memory chips) were prohibitive at the time.
None of this is to say that it could compete successfully against the C=64, but keep in mind that the Intellivision was designed as a dedicated gaming machine (to be expanded into a micro-computer later on), back in the late 70s, to compete specifically with the Atari VCS. The Intellivision was lightyears ahead of the VCS, starting with its built-in operating system with a sophisticated (if perhaps a bit limited) game engine and re-usable framework, followed by its eight built-in sprites and buffered card-oriented video display (no more chasing the beam!), all the way through its increased RAM and its three-voice polyphonic sound processor. Specifically, the buffered video was a significant step forward in capabilities.
Of course, there were things that the VCS could do that the Intellivision couldn't (those 16 colors were really limited and weird), but the Intellivision was far more capable than the VCS in almost every other practical way.
The ColecoVision was released almost 3 whole years later (the Intellivision was released on 1980, but it was test-marketed in 1979, which is proof of it being already fully developed and market-ready at that time), and took advantage of changes in the industry and hardware availability, not to mention business pressures which forced it to build upon the capabilities of the Intellivision. It is superior in some significant ways (most importantly, screen resolution), but sorely lacking in others (e.g., limits on the number of sprites that can exist on a horizontal scan-line, etc.).
It is fair to put the Atari VCS, Intellivision, and ColecoVision on the same scale inasmuch as it is recognized that they were part of an advancing continuum; and that, although they were contemporaries in the sense that they were sold during the same time period, they were products of a constantly changing industry and increasingly accessible hardware advancements. Depending on your view, you could include the Commodore 64 at the leading edge of that continuum; but by that point, the VCS is much too far behind to reasonably compare in any meaningful way.
Anyway, all that to say that the Intellivision is better than the VCS (in some important and practical aspects), somewhat favorably comparable to the ColecoVision (although at about half the screen resolution), and very much less so to the Commodore 64.
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Edited by DZ-Jay, Sat Aug 25, 2018 4:59 AM.