ARM Instruction Set Architecture Notes

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Basic instructions

(MOV) Load a immediate value into register

# represents a immdediate value.
Usage MOV register, #value
MOV R0, #10 means register0 = 10

(LDR) Load any data into register

Usage LDR register, =data
LDR R1, =0x12345678 means Load 0x12345678 into register1

(LDR) Load memory ata into register

Usage LDR register, [address]

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LDR R0, =0x12345678
LDR R1, [R0]

Move 0x12345678 into R0.
And fetch data from memory address 0x12345678 into R1.

(STR) Write register’s value into memory

Usage STR register, [address]

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MOV R0, #10
LDR R2, =0x12345678
STR R1, [R2]

move 10 into R0, and make R2 = 0x12345678
write R1’s value into R2 address’s memory.

(ADD) add value

Usage ADD register, register, (regiater or immediate value)

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Ex1:
    MOV R0, #1
    MOV R1, #2
    ADD R3, R0, R1
Ex2:
    MOV R0, #1
    ADD R1, R0, #2

(SUB) sub value

Usage SUB register, register, (register or immediate value)

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Ex1:
    MOV R0, #1
    MOV R1, #2
    SUB R3, R1, R0  ---> R3 = R1 - R0
Ex2:
    MOV R0, #1
    SUB R3, R0, #0  ---> R3 = R0 - 0

(AND) bit and

Usage AND register, register, (register or immediate value)

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Ex1:
    AND R0, R0, R1
Ex2:
    AND R0, R0, #0x23

(ORR) bit or

Usage ORR register, register, (register or immediate value)

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Ex1:
    ORR R0, R0, R1
Ex2:
    ORR R0, R0, #0x23

(B) branch

Jump to a specific address or label.
Usage B address
Sometimes we can give a label.

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    ...
    B next
next:
    ...

(BL) branch and load

Before jump to address or label, copy the next instruction’s address into R14(lr) register.
Usage same as B instruction.

ARM mode vs Thumb mode

With same code

  • Thumb uses memory space size about ARM’s 60 - 70 %.
  • Thumb’s instructions is more than ARM’s 30 - 40%.
  • Under 32 bit, ARM is fast about 40% to Thumb, but under 16 bit, Thumb is fast 40 - 50% to ARM.
  • Using Thumb code, register’s power dissipation will reduce about 30%.

Toggle b/w two mode.

(BX) Branch exchance or (BLX) Branch load and exchange.

BX and BLX is same as B and BL, but an additonal function which can switch working mode.
Usage: BX Rn and BLX Rn
If Rn’s bit [0] is 0, then processor’s mode will change(or maintain) at ARM else Thumb status.

3-Stage pipeline(ARM7)

Program Counter(PC/R15)

PC is a program counter which is R15(register15),
This register is always point to the fetched instruction.
not executing one.

Diffence when ARM or Thumb status.

Under ARM status, PC will point to current instruction + 8.
If Thumb status, PC will point to current instruction + 4.

Register

(SP)Stack Pointer - R13
(LR)Link Register - R14
(PC)Program Counter - R15
(CPSR) Current Program Status Register
(SPSR) Saved Program Status Register

Reference

ARM9