Mainframe Upgrade Done with Wire Cutters

Unrelated, but the title made me think of this video where AT&T failed over old switch frames to new ones by manually cutting all the cable bundles, very dramatic.

https://www.youtube.com/watch?v=saRir95iIWk (Speedy Cutover Service, SXS switching cutover to ESS filmed live at Glendale CA central office, 1984)

IBM mainframes these days are software-"upgradable". The number of CPUs you get to use depends on your license, and the hardware generally has hot standby extras.

Some NCR cash registers used this trick: they shipped with all the hardware included, but jumper blocks determined what percentage of the hardware the customer had paid for. As the customer’s business grew, they could purchase additional capacity, and an NCR service technician would visit to adjust the jumpers, instantly unlocking 25% more capacity. The actual limits were software-based; the jumper block was only read at startup or when a special code was entered on one of the terminals, and the software limits would adjust accordingly.

I tried describe example, how made really serious upgrades that's time.

So, once, memory appeared much cheaper than before, so one could grow from for example 16k to 64k.

What we do, we literally soldered RAM ICs over old ICs, but with trick - usually 2 highest address pins left unsoldered to PCB, but with separate wires connected to address bus, and got 64k RAM instead of original 16k.

In some later designs even appeared additional address decoder, so from for example with original Speccy 16k, we very cheap got 128k, which was incredible at that time.

I don't know exact limits of such upgrades, but seen myself z80 with 2Mb RAM and hear about 8086 with 16Mb (originally shipped with 128k).

One my buddy modified PC clone board, to boost clock from original for it 5MHz to 14MHz (he said, he have run it on 20MHz but unstable).

At diagram approximately shown, bits 14..15 of address separately handled.

  ┌──────────┐
  │          ┼────1
  │          ┼────2
  │          ┼────3
  │          ┼────4
  │          ┼────5
  │          │
  │          │
  │          │
  │          ┼─────
  │          ┼────13
  │          ┼─────14
  │          ┼─────15
  └──────────┘