PDP-8 semiconductor memory

C. H. Dickman chd_1 at nktelco.net
Tue Aug 15 18:35:01 CDT 2006

Tony Duell wrote:
>> From: "Al Kossow" <aek at bitsavers.org>
>>> http://bitsavers.org/pdf/msc/MSC3102_PDP8semiMem.pdf
>> That one is much clearer, though I cannot find the logic in it 
>> (if any) that inhibits reads when using the bootstrap cards.
> The bootstrap ROM in my 8/e is a diode matrix ROM that doesn't appear in 
> the memory map. When you flip the 'SW' switch, the boot ROM pretends to 
> be a front panel and copies itself into the appropriate bit of core.
The signal in question is ROM ADDRESS L. The MM8-E uses this as an extra 
select line, unconditionally deselecting when low. Presumably when the 
ROM decodes its address, it pulls this low, disabling all R/W memory. A 
neat way to make a hole. The MR8-E section of the maintenance manual 
confirms this.

Another effect of ROM ADDRESS L is that it prevents the PC from being 
incremented (to skip over the return address) during the execute state 
of a JMS instruction. This is stated in the Small Computer Handbook and 
in the KK8-E Processor Flow Chart. The SCH says that this is to save a 
ROM location. Wouldn't a JMP instruction be just as effective?


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