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PostPosted: Tue May 08, 2018 1:37 am 
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Posts: 426
It comes down to how they take their clocks and how they are marketed.

So a 6502 takes a a 2 phase non-overlapping clock, this basically gives it a 2mhz clock from a 1Mhz clock. This means its clock drivers are more complicated. The Z80 for example takes a 4Mhz clock, single phase. The 6510 fixed the clock issue and generates the clock internally from a single input clock. So from the 1Mhz clock the 6502 gets 4 events, from the 4Mhz clock the Z80 gets 4 events.

You can't just have something happen from nothing, you need an "event" to trigger it, and to gate the logic steps.
Say LDA #4
Phi2 Hi: You need put the PC on the address bus
Phi2 Lo: You need to read the data from the data bus
Phi1 Hi: decode the opcode
Phi1 Lo: increment the PC
Phi2 Hi: put the PC on the address bus
Phi2 Lo:read the data from the data bus
Phi1 Hi: set the A with the value
Phi1 Lo: increment the PC

Waiting cycles is not for RAM, you can just slow down the clock speed if you want to use slower RAM.

Take the Z80
Clock 0 : put PC on address bus
Clock 1 : increment PC
Clock 2 : read value from data bus
Clock 4 : Do Ram Refresh, decode Instruction
Clock 5 : put PC on address bus
Clock 6 : increment PC
Clock 7 : read value from data bus into A

If you want to do more things at once, you need to have move adders and the like on the die. So if you do it step by step you can use the ALU to increment the PC, as well as do an ADD instruction. Each bit of parallelization requires more die, which drops the number of CPUs per wafer and more gates equals higher chance of a chip failing and hence yes working chips, so that pushes the price up more.
your max clock speed is determined by which step takes the longest amount of time to do. So you might break a step down into smaller steps which makes it take longer but it allows for a higher clock overall.


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PostPosted: Tue May 08, 2018 11:50 pm 
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Joined: Tue Feb 07, 2017 2:03 am
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Sophie Wilson seems to have just given a talk about the design of the ARM https://hackaday.com/2018/05/08/sophie- ... efficient/


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