IBM 5150 maximum memory?

[Jim Battle]

Jim Leonard wrote:
> Tony Duell wrote:
>> [1] When I got my first IBM PC-family machine (a 5160), I took it to 
>> bits (what a suprise) and sat down with the TechRef. Every few minutes 
>> I'd exclaim 'They did WHAT???' as I found aother it of misdesign...
> 
> That's not the first time you've mentioned that, so now I have to ask 
> you:  What's the #1 most glaring misdesign you can remember about the 
> 5150 or 5160?

I never studied the 5150 schematics, although I once designed an ISA 
plug in card containing a 68020, DRAM, and three ASICs.  We had no ISA 
specs to design off of, so I just looked at the bus interface part of te 
PC design.

Two things come to mind, one of which was recently mentioned.  The 
interrupt structure was poor -- using edge triggered interrupts prevents 
sharing an interrupt line.  As recall the choice of edge or level 
sensitive interrupts is a matter of software (programming a control 
register in the interrupt controller chip).  There was no cost 
associated with doing it one way or the other.  Someone just decided 
incorrectly, and once the convention was set, it couldn't be undone.

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 
malicious, software could turn off the refresh and cause the memory 
state to melt.  A friend wrote a program that turned off that DMA 
channel on purpose, but made sure his program would touch every row in 
the DRAM frequently enough that his program would keep running. 
However, if anyone tried to halt the program to capture the memory image 
so they could reverse engineer his game, they'd get corrupted memory.