[cctalk] Re: Z80 vs other microprocessors of the time.

Um, no. Load-store machines like the PDP-1, or many other machines of that era, have instructions where typically one operand is a register and the other is a memory location. That means arithmetic operations necessarily include a memory reference, implying a memory wait. A key part of RISC is arithmetic on registers only, and enough registers so you can schedule the loads and stores to run concurrently with other arithmetic operations. The CDC 6600 is the pioneering example. A very simple scenario would be a memory move loop, where you'd issue two loads to two different registers, then two register-register move operations that use different functional units, followed by two store operations from two different registers. (The move operations because the 6600 would do loads to one set of registers and stores from a different set.) Keeping two memory operations in flight concurrently made quite a difference. In COMPASS: MORE SA1 A1+B2 (B2 = 2) SA2 A2+B2 BX6 X1 LX7 X2 SB3 B3-2 SA6 A6+B2 SA7 A7+B2 PL b3,MORE paul