Description

This item is from the PC Hardware FAQ, by Willie Lim and Ralph Valentino with numerous contributions by others. (v1.25).

2.26 What do wait states and burst rates in my BIOS mean?

[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

Modern motherboards are equipped with variable clocks and features for tuning board performance at each speed. The BIOS knows how to program the register bits which control these options.

1. Wait states may be adjustable to allow for slower DRAMs or cache RAMs. If you don't have a motherboard manual, or it doesn't say, then you will just have to experiment.

2. Sometimes a wait or two on a write is required with write-through cache. The programming allows for slower DRAMs. The extra wait state may cost you enough time that you would do better running at a slower clock rate where the wait state is not required.

3. Burst rates refer to the number of wait states inserted for each longword access in the cache fill cycle.

Bob Nichols (rnichols@ihlpm.ih.att.com) adds: These numbers refer to the number of clock cycles for each access of a "burst mode" memory read. The fastest a 486 can access memory is 2 clock cycles for the first word and 1 cycle for each subsequent word, so "2-1-1-1" corresponds to "zero wait states." Anything else is slower.

How fast you can go depends on the external clock speed of your CPU, the access time of your cache SRAMs, and the design of the cache controller. It can also be affected by the amount of cache equipped, since "x-1-1-1" is generally dependent on having 2 banks of cache SRAMs so that the accesses can be interleaved. With a 50MHz bus (486DX-50), few motherboards can manage "2-1-1-1" no matter how fast the SRAMs are. At 33MHz or less (486DX-33, 486DX2-66), many motherboards can achieve "2-1-1-1" if the cache SRAMs are fast enough and there are 2 banks equipped (cache sizes of 64KB or 256KB, typically).

 

Continue to:

• prev: 2.25 Can I use Mac or PS/2 SIMMs in my PC?
• Index
• next: 2.27 Cache terminology, what does it mean?

Share and Enjoy

Tags

pc, hardware, faq, frequently asked questions, repair, computer