I sometimes give seminars on the history of computing machines and like to use an Elm simulation of an 8-bit version of the Manchester Small-Scale Experimental Machine (SSEM). The one below is useable and there is a full page version here.
It has a memory of 32 8-bit words, an 8-bit accumulator and a 5-bit program counter. Unlike the original it doesn’t increment the program counter before executing the instruction so is a bit easier to describe and program.
The instruction set and encoding is the same as the original machine.
| Inst. | Description | Encoding |
|---|---|---|
| JMP S | Jump to address at S | 000***** |
| JRP S | Jump to program counter + value at S | 100***** |
| LDN S | Load and negate value at S into accumulator | 010***** |
| STO S | Store contents of accumulator at S | 110***** |
| SUB S | Subtract number at S from accumulator | 101***** |
| CMP S | Skip next instruction if accumulator is negative | 011***** |
| STP | Stop | 111***** |