Thanks David! That diagram (Apple Mac Logic) explains perfectly why the 1990 Mac Classic and the Atari ST were often 20-25% faster than the original Mac despite all being 8 MHz 68000s. The Original Mac gave 4 cycles of memory for CPU followed by 4 cycles of memory for everything else. The ST gave 2 cycles of memory for CPU followed by 2 cycles of memory for everything else (glue/shifter/mmu). The upgraded Macs basically gave 6 cycles of memory for the CPU then 2 cycles for everything else by doubling memory speed. The 68000 would stall for some (many?) instructions with the 4/4 cycle but is able to more or less run at full speed on the 6/2 (Mac) or 2/2 (ST) cycling. Interesting & Thanks!! Last question on the Mac Audio -- Is it fair to say that if you wanted audio on the Mac, you basically wanted to digitally sample a sound/take a sound sample and then have the CPU shape it so it would output correctly? (i.e. costing some CPU cycles) On Thu, Jun 24, 2021 at 11:21 AM David Riley <fraveydank@gmail.com> wrote:
On Jun 23, 2021, at 4:24 PM, John Heritage via vcf-midatlantic < vcf-midatlantic@lists.vcfed.org> wrote:
Hey folks --
Two items I'm curious about on the classic (and very original) Apple
Macs..
1. Mac (128kb ,512kb, etc) audio -- was there a custom or off the shelf sound chip for the audio on the original Mac? and was it 1 channel / and was it a custom chip by Apple? (how did it achieve speech synthesis in 1984)
It used some extra gates on the PALs in the timing and/or memory controllers to do a PWM which was then filtered down by an RC circuit to make a crude (but effective) DAC, similar to how delta-sigma DACs work. A similar circuit did the speed control for the floppy drive, which slowed down as it approached the outer tracks. Both took the PWM width data from unused bytes at the end of the video rows as the video scanning circuit went through horizontal refresh, so there weren't any real wasted cycles.
2. The Macintosh Classic released in 1990 "was about 25% faster than the original Macs". This is in line with the Atari ST often being quoted as faster than a Mac when emulating a Mac via Magic Sac or similar. What changed in the architecture to make the Mac Classic (or an ST in emulation) faster than the original Mac? Was it faster memory that gave more availability for CPU usage? were only certain instructions sped up or was everything faster?
It was actually improvements in the bus timing PALs that made the cycles a little tighter, allowing the CPU to run faster because it wasn't waiting for RAM as long. Jecel Assumpciao has/had some really good stuff on the memory timing generator (including the sound/floppy PWM) here: http://archive.retro.co.za/mirrors/homepage.ntlworld.com/kryten_droid/AppleM...
In any case, it was still an 8 MHz 68000, but the improved memory timings did yield a performance improvement.
- Dave