Sparr

A toy VM

The VM

Made up of:

• Two stacks, A and B.
• 10 general use registers r0 to r9.
• Value register rv, primarily for storing the output of operations but can also be accessed as a general register.
• Prog counter rc, the next instruction to be executed; writeable.
• Stack size register (readonly), rs.

At initalisation, all registers have the value 0 unless otherwise stated. The stack starts empty.

Program counter

Empty lines and comments are treated as no-ops so that line numbers in the source are mapped directly to the counter at runtime.

The prog counter is one-indexed (so will have the value 2 as the first operation is performed).

If the prog counter is ever set outside the bounds (<0 or >prog length) of the program then the program halts.

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Operations

Stacks and Switching

There are two stacks A and B. Only one is "active" at a time and is the target of all stack-related operations

swap

Swaps the active stack

The rs register is readonly and holds the size of the currently active stack.

Set

set <number>

set rx

Set rv to the value of the number or the value of the given register

Push

push

Push the value in rv to the stack

Pop

pop

Pop the top of the stack to rv

Copy

cp rx ry

Copy the value in register rx to register ry

Conditional Copy

cpgt rx ry rz rw - Greater than

cplt rx ry rz rw - Less than

cpeq rx ry rz rw - Equals

Copy the value in rx to ry, if rz <op> rw is true

Add/Sub/Mul/Div/Mod

add rx ry - Addition

sub <number> <number> - Subtraction

mul rx <number> - Multiplication

div <number> ry - Division

mod <number> ry - Modulus

Apply the operation to the two values (either literals or stored in a register) and store in rv. The operation is always applied as read, i.e. div 1 2 is equivalent to 1 / 2.

Output

Write

wrt rx

wrt <number>

Write the value in the given register, or the given value to the screen followed by a newline

Write Stack

wrts

Pop all values off the stack in order and print to screen on a single line

Write Character

wrtc rx

wrtc <number>

Convert the value to it's ascii equivalent and print to screen followed by a newline

Write Stack

wrtsc

Pop all values off the stack in order, convert to ascii equivalent, and print to screen on a single line