LOAD instruction

Also called: LOAD, load from memory

The instruction that copies a word from memory into the accumulator. In Tiny8, LOAD a does ACC ← M[a] and updates the zero flag.

LOAD a (Tiny8 opcode 1) copies the word at address a into the accumulator: ACC ← M[a]. The memory word is unchanged; the ACC's old value is gone.

Its execute steps:

E1 MDR ← M[MAR]: read the operand. The MAR already holds a, from the decode step.
E2 ACC ← MDR: copy it into the ACC.

With fetch and decode, that's 6 cycles.

Because the ACC loads, the zero flag loads too: Z = 1 if the loaded word is 0. That makes LOAD useful for testing a value: LOAD it, then JZ.

LOAD is how almost every Tiny8 calculation starts, since ADD and SUB need a value already in the ACC. Its partner, STORE, goes the other way.

Worked example

Example

LOAD a zero

ACC = 5, Z = 0, and M[9] = 00. Tiny8 executes LOAD 9 (19).

1.
E1: MDR = 00.
2.
E2: ACC = 0, and since the new value is 0, Z = 1.
3.
M[9] is still 00. LOAD only reads memory.

Common mistakes

Loading the number a instead of the word at address a.
Forgetting that LOAD updates Z.

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LOAD instruction is taught in Basic CPU / Computer Architecture.

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