Bus size question.

[kag]

We have an assignment due for tomorrow afternoon and we can't find the answer in our books.

Imagine a computer with a 32 bits microprocessor with 32 bits instructions; 8 bits for operation code, and the rest for either an address or an immediate operand.

What is the impact on the speed of the system with those situations:
1. A 32 bits local address bus and a 16 bits local data bus.
2. A 16 bits local address bus and a 16 bits local data bus.

 

[Dulanic]

Performance/speed would be the same since your data bus is the same. Address bus purely tells where data should go, youd just beable to access less memory.

 

[zephyrprime]

Wait a second, I'm not so sure about that. It's perfectly possible that you'd still have 32bit addressing and more than 16bit of memory even though you only have a 16bit address bus. If this were so, reading memory in the 16bit address bus would require 2 cycles to send the address and 2 cycles to receive the data back plus how ever long to actually took for the ram to get the right word of memory. If you had a 32bit address bus, you'd cut this down to: one cycle + two cycles + however long to took to get the right byte of memory. However, if you were writing to memory, you could conceivably send both the address and data at the same time and that would give you only a 2 cycle latency for writing (not counting how long it actually took to write stuff on RAM). But this would only be true if your writes were buffered by a chipset (assuming your processor even had a chipset and not an onboard memory controller like the Hammer) or if your ram were a bussed ram like Rambus and not directly controlled like SDRAM & DDR. The question as it is posed is just too simple minded and doesn't take into account enough real factors.

In short, the 16bit case would be 1 cycle slower than the 32bit case for memory reads and conceivably no penalty for the writes.

But I don't really know so don't trust me!

Hey, just so you know, the 286 & 386SX had 24 address lines and 16 data lines.