cctalk Digest, Vol 131, Issue 40

On Thu, Jul 24, 2014 at 10:22 PM, Eric Smith <spacewar at gmail.com> wrote: The 370/145 had entirely writable control store, loaded from a floppy every time the machine was powered on. I'm pretty sure the 370/15x and 370/16x also had at least partially writable control store that could also be loaded from floppy, for diagnostic purposes, and to have firmware emulation of older machines. Some models of the 360 family had alterable control store in the form of patterned cards. The 360/30 had mylar cards that could actually be punched on a keypunch to create custom microcode. These cards had one plate of a capacitor plus the dielectric material, and by punching out the plate, you made a zero at that location. The 360/50 and 360/65 had a scheme where there was a word line board and a bit line board with traces at right angles, and a solid mylar sheet between them. The bit line board traces had wide flags on either a 1 line or a zero line, depending on what the bit at that position was to be. So, the board etch pattern was changed to alter the firmware. I think most modern, high performance CPUs do not use microcode, as the two-stage instruction execution would be slower than just doing the decode in pure logic. Also, modern CPUs are so massively pipelined, and most microcode implementations are pretty much uniprocessor-like, I don't know how you'd handle all the pipelining with microcode. They may use some microcoded functions to handle special cases, though. My main point to respond to was that alterable microcode predates the VAX 11/780 by quite a bit. I'm pretty sure Burroughs also had machines with writable control store earlier. Jon