Instruction Set Design and Data types

1. Immediate Addressing : data (only 1 byte) is specified in the instruction

MOV R5, #0x20 ; 20H will be copied to R5

2. Register Addressing : data is given by registers only

MOV R5, R1 ; R1 will be copied to R5

3. Direct Addressing : address of operand is given in instruction, address will be 12 bits offset from PC given by assembler

LDR R5, Variable(LABEL) ; R5 <- [Variable]

4. Indirect Addressing : Address of operand is given by register

LDR R5, [R1] ; R5 <- [R1]

i. Register relative indirect addressing mode : address of memory operand = register + Numeric Value

LDR R0, [R1, #0x04] ; R0 <- data from mem pointed by (R1+4), here R1 remains unchanged

a)Pre-Indexing:

LDR R0, [R1, #0x04]! ; first, R1<-R1+4

then, R0 <- data from mem pointed by (R1)

b)Post-Indexing:

LDR R0, [R1], #0x04 ; First, R0 <- [R1]

then, R1<-R1+4

ii. Base Indexed Indirect Addressing: Here address of the memory operand is given by a sum of two registers

First register acts as base, second acts as index register

LDR R0, [R1, R2] ; R0 <- data form mem pointed by (R1+R2)

a)Pre-Indexing:

LDR R0, [R1, R2]! ;First, R1 <- R1+R2

then, R0<- data from mem pointed by (R1)

b)Post-Indexing:

LDR R0, [R1}, R2 ; First, R0 <- [R1]

then, R1<-R1+R2

iii. Base with scaled index indirect addressing: here address of the mem operand is given by a sum of two registers, where first reg acts as base

and second reg scaled index by shift left

LDR R0, [R1, R2, LSL #2] ; R0<- data from mem pointed by (R1 + R2 shifted left by 2 bits, R1 remains

unchanged