Alternate Assembly Syntax for ADD #5, R1 and Addressing Modes
The question asks which of the listed “alternate ways” correctly expresses the same instruction as:
- Instruction
ADD #5, R1
Here, #5 typically denotes Immediate Addressing (the constant 5 is used as the operand), and R1 denotes Register Addressing (the result is written to register R1). In many instructional assembly syntaxes, the same instruction is written using different mnemonic/operand conventions to indicate addressing modes explicitly. However, without the exact target ISA/simulator’s syntax definition, multiple candidate forms (e.g., ADDI, ADD [5],R1, ADDIME) may or may not be valid.
At a high level, these correspondences are usually the rule:
#5→ Immediate constant 5[R1]→ Indirect/Memory Indirection (memory at address in R1)[5]→ memory at absolute address 5 (not the literal constant 5)
The options provided test whether each candidate preserves the meaning “add constant 5 to the contents of R1 and place the result in R1”.
Interpreting each option
We evaluate each choice by mapping its operand syntax to addressing mode meaning.
Decide which alternate instruction matches `ADD #5, R1`
- 1Step 1
ADD #5, R1means the source operand is an immediate constant 5, and the destination operand is the register R1. - 2Step 2
This uses
[5](memory at address 5) and[R1](memory at the address held in R1). That changes both source and destination from immediate/register to memory indirection. - 3Step 3
ADDIis commonly used to indicate an add-with-immediate operation, and5(without#) is then treated as the immediate constant by the mnemonic convention. - 4Step 4
Even if
ADDIMEimplies immediate, the destination is[R1], which writes to memory pointed to by R1 rather than to R1 itself. - 5Step 5
If your ISA/syntax supports an immediate-form mnemonic (e.g.,
ADDI) then (ii) will be a valid alternate, so (iv) is unlikely.
Correct option
Given typical assembly-language syntax conventions:
- (i) is not equivalent because
[5]means “contents at memory address 5”, not literal constant 5, and[R1]means “memory at address in R1”, not “register R1”. - (iii) is not equivalent because it targets
[R1](memory indirection destination), notR1(register destination). - (ii) is the standard alternate form when the ISA uses
ADDIto denote immediate add and omits the#marker because the immediate-ness is encoded by the mnemonic itself.
✅ Therefore, the intended answer is:
- (ii)
ADDI 5,R1
Mnemonic vs operand marker
Some ISAs use # to mark immediate per operand, while others use a distinct mnemonic like ADDI and then treat the operand as immediate automatically. Always treat these as syntax-specific rules.
Don’t confuse `5` with `[5]` or `R1` with `[R1]`
[x] usually means indirect/memory access. So ADD [5],R1 reads from memory address 5, while ADD #5,R1 uses the literal constant 5. Likewise, destination [R1] updates memory, not the register itself.
How addressing-mode notation affects instruction meaning
`#value` or mnemonic-immediate (e.g., `ADDI`)
Immediate constantOperand is the literal constant used in the ALU operation."
`R1`
Direct registerOperand is the contents of register R1; destination writes back to the register."
`[R1]` or `[address]`
Indirect / memoryOperand refers to memory at the effective address; destination writes to memory."
Only forms preserving (immediate source) + (register destination) are equivalent
Equivalence testAny option changing either addressing mode changes the instruction’s effect."
Common confusion points
Knowledge Check
Which option matches the semantics of ADD #5, R1 in typical assembly addressing-mode notation?