IBM 5160:  RAM on '64 - 256KB' Motherboard


4 banks of 64 KB motherboard RAM, providing up to 256 KB of conventional memory.

RAM chips of type 4164 are used, of an access time rating of 200 ns (or less), and of the type where pin 1 is not used (Not Connected).  4164 examples.
200 ns is the figure that appears in IBM's technical reference for the 5160.

Bank 0: addresses     0 KB to   64 KB   (socketed)
Bank 1: addresses   64 KB to 128 KB   (socketed)
Bank 2: addresses 128 KB to 192 KB   (socketed)
Bank 3: addresses 192 KB to 256 KB   (socketed)

Click here for a diagram that shows the bank and bit layout on the 5160 motherboard.

RAM related switch settings on motherboard

Switches 3 and 4 on switch block SW1 selectively disable/enable the RAM banks on the 5160 motherboard.
The settings are at here.

Increasing conventional memory

Normally, but not mandatory for the IBM 5160, you would fully populate the motherboard RAM banks before adding more conventional memory via expansion cards.  An example is shown here.

However, in the IBM 5160, you have the option of adding expansion card based conventional memory without having to first populate all of the motherboard RAM banks.  An example is shown here.

Design flaw

Be aware of a design flaw, one that affects the 5160's ability to determine how much conventional memory is fitted (on motherboard + on expansion cards).
The flaw is discussed at here.

POST test of conventional memory

During the 5160's Power on Self Test (POST), the POST tests the conventional memory (the amount that it thinks is fitted).

If a problem is found in the first 16 KB (assumption: 11/08/82 BIOS), the POST will halt.  It will appear to you that the motherboard is 'dead'.  For further information, see the section below titled 'Failure of first 16 KB'.

If a problem is found in the remaining RAM, a 201 type error is reported on-screen.  The format of a 201 error produced by a 5160 is "aaaaa bb 201".  An example is "20000 80 201".
The "aaaaa" portion indicates which address the fault is at.
The "bb" portion indicates which bit/s is/are faulty.

You can use the diagram here to calculate the failing bit/s, and the address as a KB figure.

Click here for a diagram that shows the bank and bit layout on the 5160 motherboard.

640K of motherboard RAM

A modification is possible so that 640K of RAM can be fitted to the 64-256KB motherboard.
You will need certain parts, and soldering skill.  The procedure can be found here.
Be sure to keep track of what you do, so that if things do not work out, you can undo what you have done.
If you are successful, consider marking the motherboard in some way to remind yourself (and others) that the motherboard has been modified.

Failure of first 16 KB

If the POST (assumption: POST in 11/08/82 BIOS) discovers a problem in any of the first 16 KB of RAM, the POST will stop, providing no indication whatsoever (visual or audible).  To you, it will appear that the motherboard is 'dead'.

Note that the first 16 KB corresponds to the first quarter of each chip in bank 0 (64 KB sized).  RAM chips fail in different ways.  Therefore, if a RAM chip in bank 0 fails completely, you'll see the 'dead' motherboard symptom described earlier.  But if a RAM chip in bank 0 fails in such a way that the failure only affects an address/addresses in the last three quarters of the chip, then the BIOS will display a corresponding 201 error.