8 Nov
2005
8 Nov
'05
7:14 a.m.
On 11/8/05, Tim Shoppa <tshoppa at wmata.com> wrote:
Thank you. After analyzing the Qbus activity diagram I conclude that
my controller won't be fast. There are several reasons.
1. The overhead of 53C90A chip is significant. I may try 53C94/96 later.
2. The ISA DMA is slow for the getpkt(), putpkt(), getdesc() and
putdesc() functions. 80188XL won't be faster.
3. The 80386-16 is slow in executing the MSCP code written in C.
80188XL won't be faster.
To improve 1, I need a faster SCSI chip, or need to integrate it into
the CPLD. To improve 2, I need a small cache to speed up the DMA, or
need a faster microprocessor. To improve 3, I need a faster
microprocessor, and I may need to rewrite the code in assembly
language.
So the conclusion is, with the corrent design, the controller won't be
fast. I am reluctant to rewrite it in assembly language because I
don't want to divert too much away from the SIMH code. I may improve
the code to speed it up a little bit, maybe to 400kB/s. I am happy
that it can beat RQDX3.
vax, 9000
From a post I
made to the PDP-11 newsgroups in the previous
millenium:
...
Here are the peak data rates measured for read and write 64
blocks-at-a-time:
Read Write
---------- ----------
Andromeda SCDC 2.298 MB/s 1.131 MB/s
CMD CQD440 2.397 MB/s 1.525 MB/s
CMD CQD220 1.418 MB/s 0.882 MB/s
CMD CQD220A 2.088 MB/s 1.409 MB/s
DEC RQZX1 1.379 MB/s 1.097 MB/s
Viking QDT 0.846 MB/s 0.704 MB/s
DEC RQDX3 0.164 MB/s 0.161 MB/s