Timeline for Why does the Commodore C128 perform poorly when running CP/M?
Current License: CC BY-SA 4.0
7 events
| when toggle format | what | by | license | comment | |
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| Dec 30, 2018 at 18:18 | comment | added | RichF | Rainer, you are both right and wrong. The z80 runs at an "effective speed of 2.04 MHz". Right. However, it is clocked at 4.08 MHz, but only half the time. That's the sort-of-wrong part. A number of early cp/m machines ran at 2 MHz, so that in itself was not unusually slow. Later 4 MHz become the common speed, though. Note that in competition with the 6502, a 2 MHz z80 was roughly equivalent to a 1 MHz 6502. (generally 4 cycles to do very basic stuff on a z80 vs 2 cycles on a 6502) | |
| Dec 28, 2018 at 18:21 | comment | added | Tommy | @RainerVerteidiger the Amstrad is special because it does some trickery with timing signals to coerce a 6502-esque fixed memory access cadence, with memory available for video every other tick. Which does end up (approximately) rounding up every instruction to a multiple of four cycles. But other machines use various different schemes — on the Spectrum there's no rounding or other penalty while the processor is accessing RAM that video can't be in, on the MSX there's an extra cycle in every instruction fetch but everything else runs at the natural speed, etc. | |
| Dec 28, 2018 at 18:14 | comment | added | Tommy | In case it helps to dispel a myth: the shortest instruction on a Z80 takes four cycles, comprising two memory accesses (a fetch and a refresh) of two cycles each. Most other memory accesses take three cycles so the next big tranche of Z80 instructions is those that are seven cycles long, but there are fives and sixes, and various others that obviate any sort of attempt to fit everything to a pattern of 4n, or even 4 + 3n. | |
| Dec 28, 2018 at 18:14 | comment | added | Sixtyfive | Good question. Like I said, a guy came by and said so. I googled for "z80 clock divider" now and found at least one thing (cpcwiki.eu/forum/programming/z80-clock) where someone (2nd post) talks about a "divide by 4 circuit". Perhaps someone else can shed some light ... ? | |
| Dec 28, 2018 at 17:59 | comment | added | wizzwizz4♦ | Thanks for sharing this, but I don't see why you'd divide 2MHz by 4. Where did the 4 come from? | |
| Dec 28, 2018 at 17:40 | review | Late answers | |||
| Dec 28, 2018 at 17:59 | |||||
| Dec 28, 2018 at 17:24 | history | answered | Sixtyfive | CC BY-SA 4.0 |