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**ARM versions ARM architecture has been extended over several versions.**

We will concentrate on ARM7. The Slides revised from “Computer from Components”, Morgan Kaufman

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**Overheads for Computers as Components**

ARM ISA All instructions are 32-bit long (not include Thumb) Register and memory word are 32-bit Memory address is 32-bit Byte addressed Can be configured at power-up as either little- or bit-endian mode Note: There is a 64-bit ARM ISA © 2000 Morgan Kaufman Overheads for Computers as Components

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**ARM Registers r0 r8 r1 r9 r2 r10 CPSR r3 r11 r4 r12 N Z C V r5**

31 r2 r10 CPSR r3 r11 r4 r12 N Z C V r5 r13 (SP) r6 r14 (LR) r7 r15 (PC) CPSR: Current Program Status Register

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Status bits Every arithmetic, logical, or shifting operation sets CPSR bits: N (negative), Z (zero), C (carry), V (overflow). Examples: = 0: NZCV = 0110. = -231: NZCV = 1001.

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**ARM data instructions Basic format: Immediate operand: ADD r0,r1,r2**

Computes r1+r2, stores in r0. Immediate operand: ADD r0,r1,#2 Computes r1+2, stores in r0.

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**Overheads for Computers as Components**

ARM data instructions ADD, ADC : add (w. carry) SUB, SBC : subtract (w. carry) RSB, RSC : reverse subtract (w. carry) MUL, MLA : multiply (and accumulate) AND, ORR, EOR BIC : bit clear LSL, LSR : logical shift left/right ASL, ASR : arithmetic shift left/right ROR : rotate right RRX : rotate right extended with C © 2000 Morgan Kaufman Overheads for Computers as Components

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**Load/store instructions**

LDR, LDRH, LDRB : load (half-word, byte) STR, STRH, STRB : store (half-word, byte)

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**Address modes Register indirect : LDR r0,[r1]**

With two registers : LDR r0,[r1,r2] With 2nd register negated: LDR r0,[r1,-r2] With 2nd register shifted: LDR r0,[r1,-r2,LSL#2] with a base register and 12-bit offset: LDR r0,[r1,#4]

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**Address modes Register indirect : LDR r0,[r1]**

With two registers : LDR r0,[r1,r2] With 2nd register negated: LDR r0,[r1,-r2] With 2nd register shifted: LDR r0,[r1,-r2,LSL#2] with a base register and 12-bit offset: LDR r0,[r1,#4]

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**LDR and STR as pseudo-instructions**

LDR and STR may also be pseudo-instructions LDR r1, label STD r1, label R15/PC is used as the based register, and label is encode as its offset to PC They work if and only if label is within [-4095, +4095] of PC

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**Array Access Assume X in r0, i in r1, a in r2, all int type a = X[0];**

LDR r2, [r1] a = X[2]; LDR r2, [r1,#8] a = X[i]; LDR r2, [r0,r1,LSL #2] LSL: Logic shift left

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**Example: C assignments**

c = a + b; Assembler: LDR r0, a ; get value of a LDR r1, b ; get value of b ADD r3,r0,r1 ; compute a+b STR r3, c ; c = a+b

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**ARM Condition and Branch**

All instructions can be conditionally executed, testing CPSR: EQ, NE, CS, CC, MI, PL, VS, VC, HI, LS, GE, LT, GT, LE Example, execute if EQ: ADDEQ r1, r2, r3 Branch operation: B #100 Can be performed conditionally Example: BEQ Branch if equal BEQ is the B instruction with condition code EQ

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Example: if statement if (a > b) { c = a; } else { c = b; } ; compute and test condition LDR r0, a ; get value of a LDR r1, b ; get value for b CMP r0,r1 ; compare a < b BLE fblock ; if a <= b, branch to else STR r0, c ; c = a B endif else: STR r1, c ; c = b endif:

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**Conditional Instruction Implementation**

if (a > b) { c = a; } else { c = b; } ; compute and test condition LDR r0, a ; get value of a LDR r1, b ; get value for b CMP r0,r1 ; compare a < b STRGT r0, c ; if GT, c = a STRLE r1, c ; if LE, c = b

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**ARM subroutine linkage**

Branch and link instruction: BL foo Copies current PC to r14. Equivalent to MIPS jal To return from subroutine: MOV r15,r14 r15 is the PC in MIPS

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**ARM Call Convention In MIPS terms**

r0 – r3: Parameters and return value, also temporary r4 – r8, r10, r11: Saved temporary r9: Usage is Platform-dependent r12: Temporary r13: Stack pointer, preserved r14: Link register r15: The program counter

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Extra Slides

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ARM ADR pseudo-op ADR and ADRL pseudo-op generate instruction required to calculate address ADRL for long-range ADR r1, FOO ; get the addr for FOO LDR r2, [r1] ; load FOO to r1

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**ARM move instructions MOV, MVN : move (negated)**

MOV r0, r1 ; sets r0 to r1

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**ARM comparison instructions**

CMP : compare CMN : negated compare TST : bit-wise AND TEQ : bit-wise XOR These instructions set only the NZCV bits of CPSR. MIPS only has SLT, and BEQ/BNE takes two register operands

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**Additional addressing modes**

Base-plus-offset addressing: LDR r0,[r1,#16] Loads from location r1+16 Auto-indexing increments base register: LDR r0,[r1,#16]! Post-indexing fetches, then does offset: LDR r0,[r1],#16 Loads r0 from r1, then adds 16 to r1.

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