I worked for Datapoint from 1975 to 1977 as a technician in board test and repair. I started out working on power supplies, later moved to mod/demod boards (for

I worked for Datapoint from 1975 to 1977 as a technician in board test and repair. I started out working on power supplies, later moved to mod/demod boards (for the tape drives), then to processor boards. I only repaired a few of the old serial processors, these were boards sent in from existing customers for repair. The production boards were the TTL boards for 2200 and 5500. The 2200 boards used a pair of 74181 ALU chips and a pair of 7489 RAM chips for the registers. They had 121 total ICs, ran at 8MHz, and instruction decoding was performed with mostly SSI chips. The 5500 boards had 131 ICs, the ALU and registers were 74S series chips and ran at 20 MHz. Instruction decoding was done by microprogrammed ROMs. The instruction set was very similar to the Z80. While the 8008 executed the 2200 instruction set, later chips made by Intel and Zilog changed the register numbers at Datapoint's request so as to prevent them from running Datapoint's code which represented a major investment for the company. On Datapoint processors, a register/register load instruction was (in octal) 3 dst src. The later chips changed this to 1 dst src, with the instruction type (high 2 bits) having the high bit inverted from Datapoint. Dst and src were similarly changed, Datapoint having A=0, B=1, ...,(HL)=7 and the later chips having A=7, B=0, ..., (HL)=6.