Five ways to find a number: modes of addressing

By the end of this lesson

By the end you can name the five modes of addressing, explain how each one uses its operand, and work out what ends up in the accumulator for a given memory map.

Same operand, five different meanings

Here is the trap: 100 is just a number. What the processor does with it depends entirely on the mode of addressing chosen by the instruction.

The mode answers one question: where do I go to get the data?

Immediate — there is nowhere to go. The operand is the data.
Direct — the operand is the address of the data.
Indirect — the operand is the address of an address. Two steps.
Indexed — the operand plus the contents of IX gives the address.
Relative — the operand is an offset from the current instruction's address.

Same-looking operand, five different journeys.

Worked example: One memory map, four loads

Memory holds these values, and the index register contains 2:

Address	40	41	42	43	44	45
Contents	44	8	99	60	17	3

What ends up in ACC for each of these instructions?

a) LDM #40
b) LDD 40
c) LDI 40
d) LDX 40

Have a go first?

Count the arrows: immediate 0 reads, direct 1, indirect 2, indexed 1 after adding IX to the operand.

Follow the arrows

CalculateQuestion 1 · Recall

CalculateQuestion 2 · Apply

Multiple choiceQuestion 3 · Apply

Answer the 3 questions above to continue.