Addressing modes

ACompressed recap

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EA = where the operand finally is. Immediate (#c, 0 accesses), register (0), direct (EA = A, 1), indirect (EA = M[A], 2), register indirect (EA = R, 1). Displacement d(R): EA = d + R (indexed: big address in the instruction; base: address in the register). PC-relative: EA = next-instruction address + offset; offset = T − (X + L). (R)+ uses then increments; −(R) decrements then uses; step = operand size. Match constructs: constant–immediate, global–direct, pointer in register–register indirect, pointer in memory–indirect, A[i]–indexed, fields/locals–base, *p++–auto-increment, push–auto-decrement, relocatable code–PC-relative.

Table used below: M[200] = 500, M[300] = 400, M[400] = 700, M[500] = 600, M[600] = 900, M[700] = 800, M[800] = 300; R1 = 400, R2 = 200, R3 = 100.

BMerged rule card

1.2.1 Immediate, register, direct, indirect and register-indirect modes

• EA = where the operand finally is.
• Immediate #c: operand c (0 accesses). Register R: operand in R (0).
• Direct A: EA = A (1). Indirect @A: EA = M[A] (2). Register indirect (R): EA = R (1).
• Total references = fetch words + operand accesses + 1 if the result goes to memory.

1.2.2 Displacement modes: indexed, base, PC-relative; auto-increment and auto-decrement

• Displacement d(R): EA = d + R. Indexed: big address in instruction, index in register. Base: address in register, small displacement.
• PC-relative: EA = (address of next instruction) + offset; offset = T − (X + L).
• (R)+: use M[R], then R += d. −(R): R −= d, then use M[R]. d = operand size.
• Position-independent code: PC-relative branches.

1.2.3 Choosing modes: arrays, pointers, constants, relocatable code; memory references per mode

• Constant → immediate; fixed global → direct; pointer in register → register indirect; pointer in memory → indirect.
• A[i] → indexed; record field / locals → base + displacement; *p++ → auto-increment; push → auto-decrement.
• Relocatable code → PC-relative (and base register).
• Data accesses: immediate/register 0; indirect 2; all others 1.
• Element address = base + i × size.

CInterleaved GATE-style questions

Q1NAT1 markDirect vs indirect

What does LOAD R0, @800 load?

Numerical answer: ________

Q2NAT1 markDisplacement

What does LOAD R0, 300(R2) load?

Numerical answer: ________

Q3MCQ1 markRegister indirect

LOAD R0, (R1) loads:

• (A) 400
• (B) 700
• (C) 800
• (D) 600

Q4NAT2 marksPC-relative

A 2-byte branch instruction at byte address 0x0200 has an 8-bit offset field holding 0xF0 (two's complement). What is the target address in decimal?

Numerical answer: ________

Q5NAT2 marksAuto-increment walk

R3 = 100. LOAD R0, (R3)+ runs 5 times with 100-byte steps. What does the fifth load fetch? (Use the table.)

Numerical answer: ________

Q6MCQ1 markMode choice

A local variable sits 12 bytes from the start of a block whose address is in register R6. The natural mode is:

• (A) indirect
• (B) PC-relative
• (C) base with displacement 12(R6)
• (D) immediate

Q7MSQ2 marksStatements

Which are true?

• (A) Indirect addressing needs two data memory accesses.
• (B) A pointer stored in memory is dereferenced with indirect mode.
• (C) Auto-decrement suits pushing onto a stack that grows toward lower addresses.
• (D) Register mode needs one data memory access.

Q8NAT2 marksMemory references

A two-word instruction ADD @A, @B (operands: memory indirect; result written to the first operand's location). How many memory accesses in total?

Numerical answer: ________

DAnswer Vault