Jump instruction

Also called: JMP, unconditional jump, branch instruction, unconditional branch

An instruction that loads the program counter with a new address, so the next instruction comes from somewhere other than the next word.

Normally the CPU runs instructions in address order, because fetch adds 1 to the program counter. A jump breaks that order: it loads the PC with a new address, so the next fetch comes from there.

Tiny8 has two:

JMP a (opcode 5): always PC ← a. An unconditional jump.
JZ a (opcode 6): PC ← a only if Z = 1. A conditional branch.

Jumps are how programs get structure:

a backward jump repeats code, making a program loop;
a forward jump skips code;
a conditional jump chooses between two paths, like an if-statement.

In hardware, JMP's execute step is one transfer, E1 PC ← IR[3:0]: the PC's mux selects the IR's address field and LdPC = 1. So JMP takes 5 cycles. It doesn't touch the ACC or Z.

Worked example

Example

Skipping an instruction

Tiny8 runs from PC = 0, with M[D] = 7: 0: LOAD D (1D), 1: JMP 3 (53), 2: ADD D (3D), 3: STORE E (2E), 4: HALT (00).

1.
LOAD D: ACC = 7, PC = 1.
2.
JMP 3: PC = 3. The ADD at address 2 is skipped.
3.
STORE E: M[E] = 7, PC = 4.
4.
HALT. M[E] ends at 7, not 14.

Common mistakes

Adding 1 to the jump target. After JMP 3, the next fetch is from address 3 itself.
Thinking JMP changes Z or the ACC. It changes only the PC.

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Jump instruction is taught in Basic CPU / Computer Architecture.

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