Add Control Flow: JUMP and Conditional Branches

Control Flow in Bytecode

Without jumps, a VM is a calculator. `JUMP`, `JUMP_IF_FALSE`, and comparison opcodes turn straight-line bytecode into real programs with loops and conditionals.

Comparisons and Branching

Comparison instructions pop two values and push a boolean (0 or 1):

`EQ`, `LT`, `GT` produce the condition for an `if` or `while`
`JUMP_IF_FALSE addr` pops the top; if zero, sets `pc = addr`
`JUMP addr` always transfers control (skipping `else` blocks, looping back)

An `if/else` in bytecode looks like:

``` <condition> JUMP_IF_FALSE else_label <then block> JUMP end_label else_label: <else block> end_label: ```

For example, summing 1 through 10 uses a loop that jumps back to `loop_start` until the counter exceeds 10.

Forward jumps need backpatching: the compiler emits a placeholder address and patches it once the target label's offset is known. CPython's `compile.c` does exactly this for every `if` and `while`.

Design Notes

Patch jump targets only after you know the final bytecode offset. If you patch too early, inserting another instruction later shifts every following address and silently breaks branches. Compiler writers keep a list of pending patches for this reason.

For This Exercise

This exercise asks you to add branching to your VM and demonstrate a loop plus conditional. You will implement comparison opcodes and absolute jumps, including the backpatch pattern your compiler will rely on next.

Add control flow instructions to your stack-based VM in C.

Requirements:

Add EQ, LT, GT, JUMP, and JUMP_IF_FALSE opcodes to the Opcode enum
Implement EQ: pop two values, push 1 if equal else 0
Implement LT: pop two values, push 1 if first < second else 0
Implement GT: pop two values, push 1 if first > second else 0
Implement JUMP: read a 2-byte address operand and set PC to it
Implement JUMP_IF_FALSE: pop top value; if zero, jump to address
Write a bytecode program that computes the sum of numbers 1 to 10 using a loop
Write another bytecode program that demonstrates if/else: print whether a number is positive, negative, or zero
Print the VM state (PC, stack) after each instruction during execution
Show the loop and conditional branching working correctly

Test:

Show JUMP instructions changing the PC
Show conditional branching (EQ, LT, GT)
Show a loop executing multiple times
Show if/else logic selecting the correct path

Control Flow in Bytecode

Without jumps, a VM is a calculator. `JUMP`, `JUMP_IF_FALSE`, and comparison opcodes turn straight-line bytecode into real programs with loops and conditionals.

Comparisons and Branching

Comparison instructions pop two values and push a boolean (0 or 1):

`EQ`, `LT`, `GT` produce the condition for an `if` or `while`
`JUMP_IF_FALSE addr` pops the top; if zero, sets `pc = addr`
`JUMP addr` always transfers control (skipping `else` blocks, looping back)

An `if/else` in bytecode looks like:

``` <condition> JUMP_IF_FALSE else_label <then block> JUMP end_label else_label: <else block> end_label: ```

For example, summing 1 through 10 uses a loop that jumps back to `loop_start` until the counter exceeds 10.

Forward jumps need backpatching: the compiler emits a placeholder address and patches it once the target label's offset is known. CPython's `compile.c` does exactly this for every `if` and `while`.

Design Notes

For This Exercise

Add control flow instructions to your stack-based VM in C.

Requirements:

Add EQ, LT, GT, JUMP, and JUMP_IF_FALSE opcodes to the Opcode enum
Implement EQ: pop two values, push 1 if equal else 0
Implement LT: pop two values, push 1 if first < second else 0
Implement GT: pop two values, push 1 if first > second else 0
Implement JUMP: read a 2-byte address operand and set PC to it
Implement JUMP_IF_FALSE: pop top value; if zero, jump to address
Write a bytecode program that computes the sum of numbers 1 to 10 using a loop
Write another bytecode program that demonstrates if/else: print whether a number is positive, negative, or zero
Print the VM state (PC, stack) after each instruction during execution
Show the loop and conditional branching working correctly

Test:

Show JUMP instructions changing the PC
Show conditional branching (EQ, LT, GT)
Show a loop executing multiple times
Show if/else logic selecting the correct path

Topic Overview

Control Flow in Bytecode

Comparisons and Branching

Design Notes

For This Exercise

Implementation Task

Progressive Hints

Resources

Open Source

Add Control Flow: JUMP and Conditional Branches

Topic Overview

Control Flow in Bytecode

Comparisons and Branching

Design Notes

For This Exercise

Implementation Task

Progressive Hints

Resources

Open Source