GEC 4090

The 4090 was the first processor which significantly extended the 4080's original instruction set. The processor was a 32bit processor based on 8 AMD 2900 bit slice processors, and was the first British 32 bit minicomputer. The CPU instruction set was backwards compatible with the 4080, but also added 32bit addressing. The system supported up to 4Mb of main memory, but this was still the same external memory crates used by the 4082/4085, and they had to be paired up to make a 32bit wide memory bus for the 4090. However, it introduced a 16kb memory cache. The main I/O processor used was an EMC/4, External Multiplexor Channel, which could address all 4Mb of main memory, although other I/O processors could be added.