JUMP Instruction - craterdog-archives/js-bali-virtual-machine GitHub Wiki

The JUMP Instruction

The JUMP instruction tells the processor to update the current address to point to a different bytecode instruction. The resulting address depends on one of the following modifiers:

• TO NEXT INSTRUCTION
• TO label
• TO label ON EMPTY
• TO label ON NONE
• TO label ON FALSE

The operand contains the target address for the JUMP instruction in the range 0x001..0x7FF, or the value 0x000 if the target address is just the next instruction.

JUMP TO NEXT INSTRUCTION

This is a degenerate version of the JUMP instruction. It simply moves on to the next instruction. It is used when multiple labels would need to refer to the same address. The bytecode for this instruction has the form:

[00000000][00000000] note: 00000000000 is not a legal address

The following pseudo-code defines this instruction:

context.address = context.address + 1

Example Context Before Execution

Example Context After Execution

JUMP TO label

The JUMP instruction with no modifier performs an unconditional jump to the address at the label operand. The bytecode for this instruction has the form:

[00000aaa][aaaaaaaa] where aaaaaaaaaaa is the address of the label

The following pseudo-code defines this instruction:

context.address = operand

Example Context Before Execution

Example Context After Execution

JUMP TO label ON EMPTY

The JUMP instruction with an ON EMPTY modifier performs the jump to the address at the label operand if the component stack is empty. The bytecode for this instruction has the form:

[00010aaa][aaaaaaaa] where aaaaaaaaaaa is the address of the label

The following pseudo-code defines this instruction:

if task.components.isEmpty() then {
    context.address = operand
} else {
    context.address = context.address + 1
}

Example Context Before Execution

Example Context After Execution

JUMP TO label ON NONE

The JUMP instruction with an ON NONE modifier performs the jump to the address at the label operand if the component on top of the component stack is none. The bytecode for this instruction has the form:

[00001aaa][aaaaaaaa] where aaaaaaaaaaa is the address of the label

The following pseudo-code defines this instruction:

condition = task.components.pop()
if condition = none then {
    context.address = operand
} else {
    context.address = context.address + 1
}

Example Context Before Execution

Example Context After Execution

JUMP TO label ON FALSE

The JUMP instruction with an ON FALSE modifier performs the jump to the address at the label operand if the toBoolean() method for the component on top of the component stack evaluates to false. The bytecode for this instruction has the form:

[00011aaa][aaaaaaaa] where aaaaaaaaaaa is the address of the label

The following pseudo-code defines this instruction:

condition = task.components.pop()
if condition = false then {
    context.address = operand
} else {
    context.address = context.address + 1
}

Example Context Before Execution

Example Context After Execution