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IBM 5170 - Extended Memory - Maximum Amount


Falacy: Address limit of 3 MB

Some people have read that the maximum memory that can be fitted to the 5170 is 3 MB. This falacy appears to have originated from a misunderstanding of IBM's announcement letter for the 5170. In that letter is the sentence, "Both models may be further expanded to 640Kb with the 128Kb memory option or up to 3Mb with 512Kb memory options. "

Two things to note about the sentence are:
• Use of the word "or".
• IBM is writing of the cards that they supplied at the time. The "512Kb memory option" is the 512KB Memory Expansion Option, and the most of those that can be fitted is five (because of the card's switches).

So, the 3 MB is derived from:

0.5 MB  of conventional memory on the motherboard (512 KB)
2.5 MB  via five of the 512KB Memory Expansion Option
=====
3.0 MB


Possible address limit of 12.6 MB

At the web page here, a comment added by someone is:

"Also, the RAM could be taken beyond 3MB. The real limit was 12.6 MB. (I believe this was true for both types of AT.) It was possible to install more RAM than that, but due to a bug in the BIOS/motherboard implementation, a manimum of 12.6MB would be recognized."

I have not seen any corroboration of that.


Possible address limit of 15.875 MB

A memory map of the IBM 5170 is shown on page 1-8 of the IBM document at here.  As can be seen, the motherboard ROMs (U27/U47 plus optional U17/U37) not only appear in the 128 KB of address space immediately before the 1 MB address mark, they also appear in the 128 KB of address space immediately before the 16 MB address mark.

Therefore, on the face of it, this implies that extended memory is not allowed to go past address FE0000 (where the ROMs start), otherwise an address conflict would exist. Address FE0000 equates to address 15.875 MB. Take off the 1 MB start address for extended memory, and we end up with 14.875 MB maximum extended memory.

However, the memory map pointed to earlier contains "Maximum memory 15Mb", and the figure of 15 MB also appears elsewhere in IBM 5170 manuals. So, maybe 15 MB of extended memory can be fitted, but the motherboard 'ignores' the portion after addresss FE0000 so that there is not an address conflict with the motherboard ROMs.

Certainly, address FE0000 is considered by IBM to be the end of RAM. The source listing for the POST in the 5170 contains, "CHECK FOR TOP OF RAM (FE0000)".