Baby Specification
The Baby machine had the following properties:
- 32-bit word length
- Serial binary arithmetic using 2s complement integers
- A single address format order code
- A random access main store of 32 words, extendable up to 8192
- A computing speed of 1.2 milliseconds per instruction
The main store was a 32*32 bit array on a Williams Tube. In addition there were two other Williams Tubes holding special storage "registers": one held the accumulator A and the other the address of the current instruction CI (Control Instruction) and the instruction itself PI (Present Instruction). A fourth tube, the Display Tube, could be switched to provide a suitable display of the current contents of any of the Williams Tubes.
The instruction format was :
3-bit function field (bits 13 to 15) — 13-bit store address (0 to 12) — 16 bits unused
There were just seven instructions (with [S] representing the contents of the word with address S):
- A=-[S]
- A=A-[S]
- [S] = A
- If A ‹ 0, CI = CI + I (i.e. if A negative, skip the next instruction)
- CI = [S]
- CI = CI + [S]
- Halt the program
Note the surprising use of the minus operator and the complication/sophistication of using the contents of a store location rather than a store address itself as the operand in control jumps. CI had to be reset to the instruction before the next to be obeyed.
Input to the Baby was by setting sequences of bits at chosen addresses using a simple keyboard. Output was by reading the information on the Display Tube.