Addressing Modes Questions and Answers

Answer: B

The correct matching is P-4, Q-2, R-1, S-3.

• Immediate: the operand is in the instruction itself, which is how a constant is supplied (4).
• Indirect: the instruction names a location that holds the address of the operand, which is how a pointer is followed (2).
• Base with index: a base register holds the start of an array and an index register selects the element, so the address varies at run time (1).
• Base with offset/displacement: a base register holds the start of a record and a fixed offset in the instruction selects the field (3).

Options that pair indirect addressing with a record element, or immediate addressing with an array, mix up the fixed offset of a record field with the variable index of an array element.

Answer: D

The one that is not a common use of displacement addressing is Register-indirect addressing.

Displacement addressing computes the effective address as \(EA = A + (R)\): an address field plus the contents of a register. Stallings lists its three most common uses as relative addressing (R is the program counter), base-register addressing (R holds a base address) and indexing (A is a base, R holds an index).

Register-indirect addressing uses only the register's contents, \(EA = (R)\), with no displacement, so it is a separate mode, not an application of displacement addressing.

Answer: D

In indexed addressing the effective address is formed by adding a constant (the displacement or base address in the instruction) to the contents of an index register. This is the Index mode, and it is useful for arrays.

In absolute (direct) addressing the address is given in the instruction itself, indirect addressing uses the address of a location that holds the address of the operand, and immediate addressing places the operand itself in the instruction. None of these adds a constant to a register to find the address.

Answer: A

In the 8085 instruction set, ADI adds an immediate byte to the accumulator, so it uses immediate addressing (1). STA stores the accumulator at a given memory address, which is direct addressing (2). CMA complements the accumulator and names no operand, so it uses implied addressing (3), and SUB subtracts a register from the accumulator, which is register addressing (4). So the match is a-1, b-2, c-3, d-4, option (A). The other options give STA or CMA the wrong mode.