Used CPU's and motherboards
I own some "5V only" motherboards. These boards have neither a 3.3V regulator nor an assembly option for one. An example for so a kind of board is the EDOM MV020:
The fastest CPU's with 5V power supply which I own are:
- AMD 486DX2-80
- Cyrix/ST 486DX2-80
My first measurements showed an disadvantage of the Cyrix/ST-processor. The EDOM motherboard doesn't have any special support for the Cyrix-CPU (e.g. WriteBack Level1 cache) - only a general DX2-processor can be set by jumpers. So I remeasured it on a late socket 3 motherboard (PcChips SYL8884 which is similar to M921) also but I got the same results...
Measurement results
The exactly setup is documented in this LibreOffice spread sheet.
First we tested the Dhrystone performance of the both processors. Reference is always the AMD processor on the EDOM board. As a kind of double check: the AMD processor was also remeasured on the PcChips board.
The Cyrix processor has app. 9% lower integer performance on the SYL8884 motherboard in comparison to AMD. The difference on the EDOM MV020 is app. 11%. AMD has on both boards the same calculation power.
The next measurement is the performance in handling of double precision numbers:
Both processors calculate nearly with the same speed (only 1% difference). AMD has compensated the disadvantage to Cyrix which was still measured at the test with the 40MHz processors.
In the next step we check the throughput to the cache-, memory and IDE interface:
The CPU feeds the cache / memory interface with data, so the faster CPU should have (and has...) the higher throughput. I have often measured this in the past: the interface of Cyrix processors to the memory is slower than of other manufacturers.
Summary
AMD wins the match but with a minimal advantage. The 5V version of the AMD 486DX2-80 is seldom today (I assume AMD produced much more 3.3V than 5V types at this time). So ST / Cyrix can be a good and much cheaper alternative on motherboards without 3.3V regulator.
falk.richter*at*yandex.com
Last update: February 17, 2020