Re: [TUHS] pdp11 UNIX memory allocation.

The Interdata 8/32, Bell Labs' first 32-bit Unix port target, had writable microcode. It added a rather modest amount to the cost of the system as I remember (about $8K). Unfortunately, it was pretty useless. The instruction format, like many machines at the time, had opcodes and then some of the instruction bits were use to get the operands -- register, memory, offsets from registers, etc. The operand handling was implemented in hardware, and any added instructions could not use get to this operand hardware. So any new instructions that were added were pretty crippled. Moreover, the implementation in microcode was flawed. If you tried to load a word through a pointer that had a -1 in it (a common error in earlier Unices where -1 was an error return from the OS), you actually got two faults--a memory bounds error and an alignment check. Unfortunately, these faults came at slightly different times--the alignment check executed 2 or 3 micro instructions and then the bounds check arrived, reset the microcode and lost all the status information. Not only was the fault unrecoverable but the only way to clear it was to power cycle the processor! A bunch of us went to talk to the manufacturer and said, in effect, "We like your machine but we won't buy any more until this fault recovery problem is fixed". It was like talking to a bunch of cinder blocks -- they just didn't get it. Shortly afterwards the VAX arrived and the rest was history...