RAM-emu/ISA.md

The Register Machine Model

The register machine has a (in)finite number of registers which can store a 32 bit, signed integer. Two registers play a special role: the accumulator, which is register number 0 by default, and the program counter, which defaults as register number 10.

The accumulator is a special register that most operations use. The program counter keeps track of which instruction we're actually executing.

This interpreter reads a single text file with one instruction per line. Whitespaces are ignored. Lines starting with # are treated as comments and are ignored.

The input is a queue (FILO), and so is the output.

Jump operations are zero-indexed. Meaning, JUMP 0 will jump to the first instruction, and JUMP 1 to the second.

Instructions (and syntax)

All instructions that take a register as argument can also take a indirect reference as argument: This means that instred of writing READ n to read register n, if the number n is stored in a register R we can write READ *R. This will read the value n of register R, and perform READ n.

READ <register_number | *register_ref>

Reads a single number from the input queue, and stores the said number in register number <register_number>, or in the register referenced by the register number <register_ref>

WRITE <register_number | *register_ref>

Writes the number stored in register number <register_number> or in the register referenced by the register number <register_ref> in the output queue.

LOAD <register_number | *register_ref>

Writes the number stored in register number <register_number> or in the register referenced by the register number <register_ref> in the accumulator (id est, register 0).

LOAD =<constant> (load immediate)

Writes <constant> directly in the accumulator.

STORE <register_number | *register_ref>

Writes the number currently in the accumulator in the register number <register_number> or in the register referenced by the register number <register_ref>.

ADD <register_number | *register_ref>

Adds to the accumulator the number stored in register number <register_number> or or in the register referenced by the register number <register_ref>.

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ADD =<constant> (add immediate)

Adds <constant> directly to the accumulator.

SUB <register_number | *register_ref>

Subtracts from the accumulator the number stored in register number <register_number> or in the register referenced by the register number <register_ref>.

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SUB =<constant> (sub immediate)

Subtracts <constant> directly from the accumulator.

JUMP <instruction_number>

Sets the program counter to instruction number <instruction_number>, thus jumping immediately to that instruction.

JZERO <instruction_number>

Sets the program counter to instruction number <instruction_number> if and only if the accumulator has value 0.

JGTZ <instruction_number>

Sets the program counter to instruction number <instruction_number> if and only if the accumulator has value greater than 0

HALT

Stops the program.