XPost: comp.sys.sinclair, comp.sys.atari.8bit, comp.sys.apple2   
   XPost: comp.sys.cbm   
   From: merkerk@deadspam.com   
      
   Bill H wrote:   
   > Peter van Merkerk wrote:   
   >   
   >>Bill H wrote:   
   >>   
   >>>===========================================================   
   >>>This message has been cross-posted to a number of revalent usenet   
   >>>groups, so please do not start a "My system is better than your system   
   >>>flame war".   
   >>>===========================================================   
   >>>   
   >>>Relying on my memory and a cursory check on a number of websites it   
   >>>appears that the majority of Z80 cpu based computers had black and   
   >>>white displays, where as the majority of 65xx computers had a color   
   >>>display.   
   >>   
   >>I don't think that is true, at least not for home computers.   
   >>   
   >>There are plenty of (early) 6502 based system which supported only B/W   
   >>display, and there are also plenty of Z80 systems which supported color   
   >>(ZX Spectrum, MSX, Amstrad CPC, Sharp MZ series, Enterprise...etc).   
   >>   
   >>If we include CP/M computers (which are Z80 based) then what you are   
   >>saying might be true. However those where intended for business use. In   
   >>those days color was not deemed to be important for business computers.   
   >   
   > I probably should have specified a time period. I was thinking more of   
   > the 78 - 82 time period and computers that were available for home use   
   > mainly in the US (only because that is where I am and have the most   
   > experience with). During that time period the popular computers on the   
   > Z80 side where the Tandy's (and clones), the zx81, ts1000 and some CP/M   
   > machines. On the 65xx side where the Apple ][s, Atari's and Commodores.   
      
   In that time period there was also the Commodore PET which didn't   
   support color. I'm pretty sure there were more B/W 6502 based system in   
   that era on the market, though I don't know about the US situation.   
      
   [snip]   
   >>>So the question is, did the cpu influence the display?   
   >>   
   >>No, at least not whether the system supported color or not. Of cource   
   >>the CPU is one, but not the only, factor that affects how fast a display   
   >>can be updated, but that is another story.   
   >   
   > One reason I was thinking that the cpu could influence the display is   
   > that z80 had a larger instruction set that allowed you to do more with   
   > the cpu than the 65xx.   
      
   The Z80 had a richer instruction set than a 6502. But that doesn't mean   
   that the Z80 could do things a 6502 could not. However on 6502 it may   
   very well take more instructions to do a certain task than it would take   
   on the Z80. On the other hand the 6502 required on average less clock   
   cycli per instruction. However Z80 based systems usually run at a   
   significantly higher clock rate. This makes speed comparisions between   
   Z80 and 6502 based systems tricky; a 3.5 MHz Z80 isn't 3.5x as fast a 1   
   MHz 6502, it is generally believed that it is closer to 2x.   
      
   > So on the 65xx side you needed more support   
   > chips to do things that the cpu could not.   
      
   The Atari 2600 console (which uses a 6507), the CPU had very little   
   support from the support chips; every display line it had to continuosly   
   feed the "display chip" with new data, since the display chip wasn't   
   able to get the display from memory by itself (like the display chips in   
   most systems). So if speed isn't a concern a 6502 based system doesn't   
   necessarilly require more support chips than a Z80. If speed is a   
   concern, having support chips to offload the CPU always helps, even on a   
   Z80 based systems.   
      
   The Z80 did have feature that 6502 did not; the Z80 did have support for   
   DRAM refresh so it could be used with DRAM memory with minimal support   
   circuitry. On the C64 the video chip (VIC-II) is responsible for the   
   DRAM refresh   
      
   > For example on a 65xx would it be possible to drive the display as is   
   > done on the zx80 (not the ts1000 or zx81 which used a custom chip) and   
   > the cpu with only a few off the shelf logic chips?   
      
   The ZX80 to advantage of the DRAM refresh and some other Z80 specific   
   "features" to implement a very simple (but clever) display system. Some   
   aspects like executing a display file might be possible on a 6502 based   
   system, other aspects would require another solution.   
      
   On the other hand the 6502 had a feature that allowed display chips to   
   use the same memory as the CPU without slowing the CPU down (at least   
   most of the time). The 6502 accessed memory only one phase of the clock,   
   the other phase could be used by the display chip without the CPU   
   noticing it. Of course the display chip had to be specificly designed to   
   be matched with the 6502, which may explain why 6502 based system often   
   use custom designed chips, whereas many Z80 based systems used   
   of-the-shelf solutions.   
      
   --- SoupGate-Win32 v1.05   
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