let's see if I'm allowed to post to Lisa list today. I don't know about all that. Could an early Mac do this:
http://www.oldskool.org/pc/8088_Corruption --- Ray Arachelian <[EMAIL PROTECTED]> wrote: > 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. > > -- > LisaList is sponsored by <http://lowendmac.com/> > and... > > Shop buy.com and save. > <http://lowendmac.com/ad/buy.com.html> > > Support Low End Mac > <http://lowendmac.com/lists/support.html> > > LisaList info: > <http://lowendmac.com/lists/lisa.html> > --> AOL users, remove "mailto:"; > Send list messages to: > <mailto:lisalist@mail.maclaunch.com> > To unsubscribe, email: > <mailto:[EMAIL PROTECTED]> > For digest mode, email: > <mailto:[EMAIL PROTECTED]> > Subscription questions: > <mailto:[EMAIL PROTECTED]> > Archive: > <http://www.mail-archive.com/lisalist%40mail.maclaunch.com/> > > iPod Accessories for Less > at 1-800-iPOD.COM > Fast Delivery, Low Price, Good Deal > www.1800ipod.com > __________________________________________________ Do You Yahoo!? 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