IBM 5150 maximum memory?

Chuck Guzis cclist at sydex.com
Sun Apr 27 01:15:04 CDT 2008


Date: Sat, 26 Apr 2008 20:58:36 +0100 (BST)
From: Tony Duell

> Another silly thing is that refresh was controlled by a DMA channel. I'm
> sure it saved a couple chips, but it meant that errant, or 
> 
> And, indeed, using the 8237 DMA chip with a paging register (and not even
> doing that as elegantly as the FTS-88 did, which at least had one paging
> register per DMA channel) rather than using the 8089 'I/O processor'. 

The 5150 has 4 page registers, one for each DMA channel (an LS670 4x4 
RAM).  Of course that limits one to doing DMA inside of 64K physical 
blocks, but that's not too awful.  The DMA-driven RAM refresh earned 
some writers beer money as they could write articles on how to alter 
the refresh rate to squeeze a bit more performance out of the basic 
box.  On the PC-AT, the big design flaw in the DMA circuitry to me 
was the omission of handshaking for DMA memory-to-memory transfers.  
It would have made the memory above 1024K much more useful on the 
system without having to switch into protected mode.

The "use DMA for refresh" wasn't an awful compromise.  At least the 
video adapters had their own private memory and didn't load the 
system down refreshing out of system RAM (i.e., IBM didn't use an 
8275 CRTC, thankfully).  Given that the 5150 used 16K DRAMs on the 
planar, the Intel memory controller would have been the 8202, a 
miserable piece of silicon.   It would also have made upgrading to 
64K DRAMs a bit more of a chore (requires an 8203).

The 8089 was pretty much of a dead-end product; limited to 20 bits of 
addressability, expensive, with only 2 DMA channels.  I never did 
figure out why Intel introduced it.  Our Intel sales engineer didn't 
even want to talk about it. 

I only vaguely remember the tamper-proof stuff on the 5150 PSU since 
I dug into it only a few days after I had the system.  Reversed the 
fan--again, it was incredible that the case interior was kept at 
negative pressure.  Suck all sorts of crud in through the floppy 
slots.  I added a filter over the fan port on the case.

I was still doing the same thing 20+ years later.  In an odd twist of 
events, I also like to replace the cheap Chinese DC fans with nice 
Japanese AC line-powered fans.  I've never had one of the latter 
develop fan noise--they're just better built.

Cheers,
Chuck




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