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P/M graphics unused memory after PMBASE


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On 12/17/2022 at 10:17 AM, Ecernosoft said:

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Now, reminder that when the 800/400 came out in '79 it was the MOST POWERFUL COMPUTER out there, except maybe the TI-99/4, but that didn't have the game library of the 800/400.

While other computers had color graphics (The AII and the TI-99/4), they didn't have sprites (Minus the TI-99/4) or a 128 color palette that if I'm not wrong, would get upgraded to 256 colors. None of these computers (Before '79) could do what the Atari 800 does easily. And- the best feature in my oppinion- ANTIC's 16 modes. Other computers didn't have that.

And while I agree with you on the C64 being superior, that CPU is slow. Sure, the ANTIC DMA can get a little crazy but even at the worst the CPU in the 800/400 seemingly runs at 1.074 mhz, STILL FASTER than a C64 WITHOUT video.Most of the time it is much faster however, usually running at about 2600 speeds or more. 

Plus. the C64 can't do multiple modes at once and doesn't have an easy Scanline counter based interrupt or 256 colors. Just saying.

The TI-99/4 (or the 4A) was powerful in the sense of having a 16-bit minicomputer-type CPU with some advanced (for the time) features, compared to the very simple, basic 6502, but there was a lot that held it back from being a higher-performance computer, such as its memory architecture.  It only had direct access to the system ROM and, if I remember correctly, 256 bytes of RAM for its registers.  The rest of its usable memory had to go across the 8-bit bus to the VDP RAM that it shared with the TMS9918 video chip.  This chip was everywhere, from MSX computers to the ColecoVision, and except for the fact that it could handle 32 sprites, it wasn't a patch on the A8's capabilities (or those of the C64, for that matter).  The sprites could be either 8x8 or 16x16 (or expanded to twice their width and height), and the main limitation was that only 4 could appear on each scan line at a time.  The hardware took care of this to a degree, and the rest was handled by software that multiplexed the sprites by flickering them (not ideal).  For the kind of games it was used for, especially on the ColecoVision, it did a good job, even competing favorably in many cases against the Atari 5200, but I think A8 programmers would have found its graphics capabilities, aside from sprites, rather limited for computer use or different kinds of games.  It was definitely not the most powerful 8-bit home/personal computer at the time, the A8 was (albeit the Apple II had some of its own advantages of a different nature).

 

The GTIA can display 256 colors, but only in a certain mode: one of the so-called GTIA modes, GRAPHICS 9, which offers 16 shades of 1 hue selected from the 16 hues available.  You'll have to use DLIs to change the hues so that you can get all 256 colors on the screen at once.  In all other modes, including the ones most commonly used for gameplay and productivity, the palette is still 128 colors (8 shades of 16 hues).  That's still way more than any other computer had at the time.

 

I for one wouldn't say the C64 is superior!  The A8 and C64 have some striking similarities but also equally striking differences, and both can do things the other can't quite do as well.  Broadly, the A8 has a lot more shades of colors, while the C64 is better at "hi-res".  The A8 got to this level way earlier, but then again the C64 did it what did more cheaply (which means, in a way, more efficiently, from an engineering and business perspective).

 

As for CPU speed, the A8 is maybe roughly 13% or so faster, effectively, depending on the graphics mode(s).  Its design is definitely more efficient in how it uses the available memory cycles, but then again it requires a 2 MHz CPU without offering 2 MHz speed (like the BBC Micro and C128 do).  About the fastest it can effectively run is about 1.65 MHz (accounting for the DRAM refresh cycles), albeit that's with the display off.  Obviously, with a decent graphics mode, the effective speed drops way down, but it's still always faster than the C64's 1 MHz CPU.

 

As for the C64 and multiple modes, if you mean a split screen, it can absolutely do that, and it's based on a scan line number comparator.  You tell it the scan line on which to generate a "raster interrupt" and that's exactly what it does.  In fact, while not as nifty as the way the ANTIC in the A8 does this in a Display List, it's really easy to set up and chain multiple raster interrupts together.  You have to do that with DLIs anyway, since there is only a single vector.  You also don't have to be at the end of a row (often multiple scan lines) to get an interrupt, so you won't have to do any busy waiting (STA WSYNC is effectively a busy-wait) to get where you need to go in most cases.   Additionally, the scan line counter register on the VIC-II is full resolution instead of half like WCOUNT.  Doing DLI-type stuff on the C64 is easy, and this includes switching graphics modes (the CPU has to do it instead of the VIC-II by itself, but it gets the job done).

Edited by Robert Cook
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On 5/23/2024 at 6:46 PM, Robert Cook said:

The TI-99/4 (or the 4A) was powerful in the sense of having a 16-bit minicomputer-type CPU with some advanced (for the time) features, compared to the very simple, basic 6502, but there was a lot that held it back from being a higher-performance computer, such as its memory architecture.  It only had direct access to the system ROM and, if I remember correctly, 256 bytes of RAM for its registers.  The rest of its usable memory had to go across the 8-bit bus to the VDP RAM that it shared with the TMS9918 video chip. 

And just to compensate for it, they invented an interpreted language (GPL "Graphics Programming Language") for programs put into VDP RAM. This design was awful enough to let TI repeat it with the TMS34010 chip which neither has direct access to the frame buffer. Instead, they put the CPU into the video chip. A rather nice CPU, even.

 

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