— Instruction: nop

Does nothing! Used for padding other instructions to certain alignments.

— Instruction: halt

Exits the VM, returning a SCM value. Normally, this instruction is only part of the “bootstrap program”, a program run when a virtual machine is first entered; compiled Scheme procedures will not contain this instruction.

If multiple values have been returned, the SCM value will be a multiple-values object (see Multiple Values).

— Instruction: break

Does nothing, but invokes the break hook.

— Instruction: drop

Pops off the top value from the stack, throwing it away.

— Instruction: dup

Re-pushes the top value onto the stack.

— Instruction: void

Pushes “the unspecified value” onto the stack.