This was a fantastic read! I had a couple of question about the following two passages:
>" Each word (A or B) has a drive line that passes either through a transformer (for a 1 bit) or around a transformer (for a 0 bit). "
What is actually "driving" these drive lines? What is the source?
>"In this model, different microcode was loaded from a card deck or tape to switch operating modes between System/360 and emulation of the legacy IBM 1400 series."
Was the System/360 the successor to the IBM 1400 series then? I'm guessing this was this the first big commercial "backwards compatible" instruction sets?
Thanks for the questions. The drive lines are activated by a matrix of source and sink drivers. The idea is that instead of 256 drivers, you use 16 sources and 16 sinks so each combination drives one line and you save a lot of components. To make this work, you need a bunch of diodes, which are in the metal cans on the board. (Footnote 2 has a diagram which might clarify all this.)
As for the importance of the System/360, you need to consider IBM's computers in the early 1960s. They had scientific computers, business computers, plus low-end computers like the 1401 for business and 1620 for science. These computers were entirely incompatible, with different instruction sets, word sizes, and architecture. As software projects got larger, this turned into a nightmare since you didn't want to rewrite your operating system for every computer.
The IBM System/360 solved this problem by using the same instruction set and architecture for all their computers, from the low end to the room-filling. The System/360 covered the whole circle of application (i.e. 360°) from business to scientific. This was a revolutionary idea at the time and became very popular. So popular that IBM's current mainframes still support the 360 architecture.
One problem was how to move customers from their old computer to the System/360 since they wanted to keep their old software. This was a big problem for IBM until someone realized that using microcode, you could emulate the old instruction set without much effort. Thus, you could buy emulation support on your System/360 computer and get the microcode to run your old 1401 or scientific programs.
Most of IBM's product numbers are annoyingly random, but sometimes marketing gets it right, as in the case of the System/360. Amusingly, the aerospace version of System/360 is called System/4 Pi: these computers are operating in three dimensions, and just as a circle has 360 degrees, a sphere has 4π steradians.
IBM also had System/370 which was the 1970s version of the 1960s System/360, and similarly System/390 in the 1990s.
>" Each word (A or B) has a drive line that passes either through a transformer (for a 1 bit) or around a transformer (for a 0 bit). "
What is actually "driving" these drive lines? What is the source?
>"In this model, different microcode was loaded from a card deck or tape to switch operating modes between System/360 and emulation of the legacy IBM 1400 series."
Was the System/360 the successor to the IBM 1400 series then? I'm guessing this was this the first big commercial "backwards compatible" instruction sets?