English Amiga Board - View Single Post - 68020 performance on an A500 and bad magazine articles

No.3 · 12 February 2024, 23:35

The 68000 has 32 bit registers and a 32 bit instruction set (but the ALU is only 16 bit). The data bus is only 16 bits wide i.e. when e.g. working with 32 bit data, the 68000 has to do two 16 bit RAM accesses to piece together the 32 bit data.

The 68020 is a full 32 bit processor i.e. also the ALU is 32 bit and the data bus is 32 bit. As the A500 memory is only 16 bit wide there are, again, two 16 bit RAM accesses required to piece together the 32 bit.
When working with 16 bit data it doesn't seem to matter (for the 68000 as well for the 68020) but it does, as optimized software will read/write two 16 bit data at once i.e. instead of two 16 bit memory accesses it will do one 32 bit memory access which is quite a lot faster!

=> It is brain dead to run a 68020 without 32 bit memory, but I know, there were a few A500/2000 turbo cards without 32 bit memory.

12 February 2024, 23:35	#7
No.3 Registered User Join Date: Sep 2022 Location: Switzerland Posts: 119	The 68000 has 32 bit registers and a 32 bit instruction set (but the ALU is only 16 bit). The data bus is only 16 bits wide i.e. when e.g. working with 32 bit data, the 68000 has to do two 16 bit RAM accesses to piece together the 32 bit data. The 68020 is a full 32 bit processor i.e. also the ALU is 32 bit and the data bus is 32 bit. As the A500 memory is only 16 bit wide there are, again, two 16 bit RAM accesses required to piece together the 32 bit. When working with 16 bit data it doesn't seem to matter (for the 68000 as well for the 68020) but it does, as optimized software will read/write two 16 bit data at once i.e. instead of two 16 bit memory accesses it will do one 32 bit memory access which is quite a lot faster! => It is brain dead to run a 68020 without 32 bit memory, but I know, there were a few A500/2000 turbo cards without 32 bit memory.