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The 6502 didn't need cache because its clock speed was slower than the DRAM connected to it. That made memory accesses very inexpensive. The Apple II took adva
by jdblair 3y ago
The 6502 didn't need cache because its clock speed was slower than the DRAM connected to it. That made memory accesses very inexpensive.
The Apple II took advantage of the speed difference between CPU and DRAM and designed the video hardware to read from memory every other memory cycle, interleaved with CPU memory access.
- Vogtinator 3y ago> The Apple II took advantage of the speed difference between CPU and DRAM and designed the video hardware to read from memory every other memory cycle, interleaved with CPU memory access. Same for the C64. Sometimes it was necessary to read slightly more than that for video display though, so the VIC (video chip) had to pause the CPU for a bit sometimes, resulting in so called "badlines".
- smolder 3y agoI think they used SRAM and not DRAM, which is how it was faster than a clock cycle.
- Gracana 3y agoNope, they used 16k x 1-bit DRAM ICs.
- monocasa 3y agoBoth were faster. DRAM was a little cheaper, but required more circuitry to handle refresh on most CPUs making it a wash cost-wise on some designs. Typical woz engineering got the cost of the refresh circuitry down to where DRAM made sense economically on the Apple ][. Interestingly Z80s had DRAM refresh circuitry builtin which which was one reason for their prevalence.
- peterfirefly 3y agoScreen output was the DRAM refresh. And for the Z80: also that it only needed GND and 5V. The 8080 also needed 12V. And the Z80 only needed a single clock phase -- the 8080 needed two. The 6502 also only needed 5V and a single clock input (the 6800 needed two clock phases). The 6502 and Z80 were simply a lot easier to work with than most of the competition.