The Calcutron-33 Instruction-Set

Coverage of all the Calcutron-33 assembly instructions

Vintage Olivetti mechanical calculator

If you want to program the Calcutron-33 CPU, my made up decimal number based RISC-like microprocessor, you need to know what instructions it supports and how they work. To get you started, let us look at a simple example of a Calcutron-33 program. It contains several instructions such as INP, BGT and JMP. In addition, there are labels such as loop, second and first. The latter are not instructions, but ways to refer to different locations in the program.

loop:
    INP  x1
    INP  x2
    BGT  x1, x2, first

second:
    OUT  x2
    JMP loop

first:
    OUT  x1
    JMP loop
    HLT

This program keeps reading pairs of numbers from its input using the INP instructions. The read numbers are stored in the x1 and x2 registers, respectively. Next, we compare the numbers in the x1 and x2 registers using the BGT instruction. BGT is shorthand for: "Branch if Greater Than." If the first operand x1 is greater than the second operand x2, the program jumps to the first label. Here there is an instruction OUT x1 which writes the contents of the x1 register to output.

BGT is what we call a conditional branch instruction. Conditional branch instructions evaluate a condition and if the condition is true, the microprocessor will jump to a…