Hardware races

Something tells me you'll be disappointed if you expect a PDP-whatever to keep up with a 486-66. If you stop and think about it, it's the rate at which a CPU gets in and out of memory that determines how fast it performs, and the memory for which the PDP-series processors were designed was quite a bit different from what was commonly used with a 486DX2. The typical '70's memory had an access time on the order of half-a-microsecond, while the typical '486-motherboard had 10-15 ns CACHE on it, together with 60-70 ns (OK, maybe as slow as 80ns) main memory. That suggests that data can be moved in and out of main memory at a rate of 12.5 MHz x 32bits, or at 50 MB/sec, and into and out of cache at nominally 246 MB/sec, in bursts. I don't know what the PDP's themselves can do, but the fastest memories I remember seeing in quantity during the late '70's were of 45-55 ns speed, though I'm sure there were some smaller faster ones. The popular types were 2147's and 2148's, organized as 4k x1 or 1k x4, respectively, though later in the '80's there were some larger ones of comparable speed avaialble. Were the PDP-types cached? What sorts of caches did they use? The Q-Bus cycle timing, the only real contact I had with the PDP-types, though my experience was with a micro-Vax, certainly didn't suggest it would operate at speeds approaching those cache speeds, so the cache would have had to be on the CPU cards. I'd submit that a fair test would be, say, to calculate the value of PI, or some other task which involves only integer computations, to, say, 250 places, using code written for each processor in assembly language, thereby eliminating the OS or a compiler as an influencing factor. Be sure system functions are turned off while it runs, so only the test process is running, and use system calls only for display and input functions. I'd say a more reasonable test would be a PDP-type against, say, a comparably equipped '286. The last one I used ran at 25 MHz. The most common ones late in their market life ran at 16, and the first ones ran at 6 or 8, depending on what you wanted to spend. In the comparisons I saw run while I was in the aerospace industry, the MicroVax-II compared more or less, with a PC/AT-clone at 12.5 MHz, just for reference, but the PC disk subsystem was much slower than the uVax's so the test was skewed. I've no idea how the uVax-II compared with the 11/34 you have in mind. Dick ----- Original Message ----- From: "Mike Kenzie" <KenzieM(a)sympatico.ca> To: <classiccmp(a)classiccmp.org> Sent: Monday, September 17, 2001 9:33 PM Subject: Hardware races