computer video (was Re: Free IBM AT)

From: Ray Arachelian <ray_at_email.domain.hidden>
Date: Tue, 28 Feb 2006 22:38:49 -0500

Chris M wrote:

>But Apple made a mistake with
>the Mac by not supplying a distinct video ic, allowing
>the 68k to do all the work, and therefore was lacking
>in speed.

That's not quite true. Both the original Mac and the Lisa shared memory access with the video hardware. The video hardware was actually much simpler than what most computers used a dedicated display chip for. It was basically a nothing more than a shift register that walked memory and spat out video signals.

Half the time the CPU had access to the memory bus, the other half the video system.

Other contemporaries of the time may have used a dedicated IC to do the video, *BUT* in most cases, these also shared access to memory with the CPU. So it was no better. Infact, they were more complex because they were text mode (40x25 or 80x25) and needed a character generator ROM. The video IC would read a byte from main memory, then turn around an read the bitmaps for that character from a ROM and display that.

I remember there were various tricks done to get various styles displayed too. For the Commodore line, there were several bitmaps (aka fonts today) that implemented primitive graphics. The high bit (128) was used to invert the bitmap, so the scheme to display the cursor was to use XOR 128 on and off every second to flash the character. There was a patent for this simple scheme. Other displays used another chunk of memory that mapped along with the text to implement attributes such as underline, flash, inverse, and another set for color.

Things like the VIC20 and Commodore 64 had some dedicated hardware to do sprites and such, it's true, but for normal operations, it wasn't too much better what the Mac/Lisa had. There were of course vector systems out there, but these were mostly for games and worked in a totally different way than raster displays like om the Mac, Lisa, Commodore's, and PC's.

Even so, they generally had to share the memory with the CPU, so there was a slowdown due to that. This can be exposed on the Commodore 128 by going into FAST mode which ran at 2Mhz instead of the usual 1Mhz. The 40column display would be shut off. (The 80 column one which ran off a chip similar to the CGA controller still worked.) Even the lowly TS/1000 had a fast mode that disabled the video because it too shared it's small memory with the video system.

I don't recall whether you had to do special stuff to access IBM PC's video memory on the CGA cards, perhaps it was accessible in memory though the video ram as it lived on the ISA card, but I do recall it displaying snow if you directly wrote to the video memory and didn't use the INT21 routines in the BIOS. Lots of program wrote directly to the screen for speed, but had to do so in the vertical retrace. (The BIOS routines were very slow.)

The Lisa ran at 5MHz even though the 68000 was an 8MHz cpu due to the video circuitry needing access to memory. I'm not sure how they fixed this for the original Mac. Perhaps faster RAM, or more likely the smaller screen real estate did the trick. In some ways, if you look at the Mac and the Lisa, the Lisa actually had something like 5 CPU's (68000, 6504, COPS, COPS in keyboard, and an optional AMD/TI FPU for the early I/O boards, and a Z8 in the Profile/Widget). The Mac had to rely entirely on the 68000.

They could have added one more CPU just to do graphics, but, that would have added a lot more expense and complexity. Besides, in that sort of system, whenever the main CPU would need to transfer a big chunk of data to the graphics controller instead of just instructions that say, draw a line from this point to that point in this color, there would be a bottle neck there.

Also, back then having a dedicated video processor didn't mean you could do graphics primitives with it. i.e. the chips did not have the silicon to draw lines, boxes, in "hires" bit mapped display modes. Rather the CPU had to do that work and there were various algorithms for it. QuickDraw just happened to be a better implementation that all of those. :-)

I'm not sure many computers had video chips that could offload graphics work from the main CPU at that stage (i.e. hardware accelerated graphics), except maybe perhaps for the Amiga, but that came later on. Most were just good old fashioned frame buffers in bit mapped mode, and character generator based displays.

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Received on 2006-02-28 19:41:56

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