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ARM Instruction Set

ARMAdvanced RISC Machines

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Stack Processing• A stack is usually implemented as a linear data structure which grows

up (an ascending stack) or down (a descending stack) memory

• A stack pointer holds the address of the current top of the stack, either by pointing to the last valid data item pushed onto the stack (a full stack), or by pointing to the vacant slot where the next data item will be placed (an empty stack)

• ARM multiple register transfer instructions support all four forms of stacks– Full ascending: grows up; base register points to the highest address

containing a valid item– empty ascending: grows up; base register points to the first empty location

above the stack– Full descending: grows down; base register points to the lowest address

containing a valid data– empty descending: grows down; base register points to the first empty

location below the stack

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• The ARM architecture uses the load-store multiple instructions to carry out stack operations.

• The pop operation (removing data from a stack) uses a load multiple instruction; similarly, the push operation (placing data onto the stack) uses a store multiple instruction.

• When using a stack you have to decide whether the stack will grow up or down in memory. A stack is either ascending (A) or descending (D). Ascending stacks grow towards higher memory addresses; in contrast, descending stacks grow towards lower memory addresses.

• When you use a full stack (F), the stack pointer sp points to an address that is the last used or full location (i.e., sp points to the last item on the stack). In contrast, if you use an empty stack (E) the sp points to an address that is the first unused or empty location (i.e., it points after the last item on the stack).

• There are a number of load-store multiple addressing mode aliases available to support stack operations (see Table). Next to the pop column is the actual load multiple instruction equivalent.

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For example, a full ascending stack would have the notation FA appended to the load multiple instruction—LDMFA. This would be translated into an LDMDA instruction.

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Example 20The STMFD instruction pushes registers onto the stack, updating the sp. Figure shows a push onto a full descending stack. You can see that when the stack grows the stack pointer points to the last full entry in the stack.

PRE r1 = 0x00000002r4 = 0x00000003sp = 0x00080014

STMFD sp!, {r1,r4}

POST r1 = 0x00000002r4 = 0x00000003sp = 0x0008000c

NOTE : Stack pointer points to the last full entry in the stack.

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Example 21In contrast, Next figure shows a push operation on an empty stack using the STMED instruction. The STMED instruction pushes the registers onto the stack but updates register sp to point to the next empty location.

PRE r1 = 0x00000002r4 = 0x00000003sp = 0x00080010

STMED sp!, {r1,r4}

POST r1 = 0x00000002r4 = 0x00000003sp = 0x00080008

NOTE : SP to point to the next empty location.

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Block Copy Addressing

Stack ExamplesSTMFD sp!,

{r0,r1,r3-r5}

r5r4r3r1r0S

P

Old SP

STMED sp!,{r0,r1,r3-r5}

r5r4r3r1r0

SP

Old SP

r5r4r3r1r0

STMFA sp!,{r0,r1,r3-r5}

SP

Old SP 0x400

0x418

0x3e8

STMEA sp!,{r0,r1,r3-r5}

r5r4r3r1r0

SP

Old SP

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Load-Store Instructions

• Three basic forms to move data between ARM registers and memory– Single register load and store instruction

• A byte, a 16-bit half word, a 32-bit word

– Multiple register load and store instruction• To save or restore workspace registers for procedure entry and exit• To copy clocks of data

– Single register swap instruction• A value in a register to be exchanged with a value in memory• To implement semaphores to ensure mutual exclusion on accesses

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• The swap instruction is a special case of a load-store instruction. It swaps the contents of memory with the contents of a register.

• This instruction is an atomic operation—it reads and writes a location in the same bus operation, preventing any other instruction from reading or writing to that location until it completes.

• Swap cannot be interrupted by any other instruction or any other bus access. We say the system “holds the bus” until the transaction is complete.

Single Register Swap Instruction

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Syntax: SWP{B}{<cond>} Rd,Rm,[Rn]

Rd <- [Rn], [Rn] <- Rm

Rm Rd

Rn

32

1

temp

Memory

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Example 21The swap instruction loads a word from memory into register r0 and overwrites the memory with register r1.

PRE mem32[0x9000] = 0x12345678r0 = 0x00000000r1 = 0x11112222r2 = 0x00009000

SWP r0, r1, [r2]

POST mem32[0x9000] = 0x11112222r0 = 0x12345678r1 = 0x11112222r2 = 0x00009000

This instruction is particularly useful when implementing semaphores and mutual exclusion in an operating system. You can see from the syntax that this instruction can also have a byte size qualifier B, so this instruction allows for both a word and a byte swap.

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0X36197488

0X09059945

0X12345678 0X00009000

0X00009004

0X00009008

PRE mem32[0x9000] = 0x12345678r0 = 0x00000000r1 = 0x11112222r2 = 0x00009000

0X54233083 0X00009008

POST mem32[0x9000] = 0x11112222r0 = 0x12345678r1 = 0x11112222r2 = 0x00009000

0X36197488

SWP r0, r1, [r2]

0X09059945

0X11112222 0X00009000

0X00009004

0X00009008

0X54233083 0X00009008

0X00000000r0

0X11112222r1

0X00009000r2

0x12345678r0

0X11112222r1

0X00009000r2

LOAD

STORE

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Concept of SEMAPHORE

• In computer science, a semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel programming environment.

• A semaphore, in its most basic form, is a protected integer variable that can facilitate and restrict access to shared resources in a multi-processing environment.

• The two most common kinds of semaphores are counting semaphores and binary semaphores. Counting semaphores represent multiple resources, while binary semaphores, as the name implies, represents two possible states (generally 0 or 1; locked or unlocked).

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• A semaphore can only be accessed using the following operations: wait() and release().

• wait() is called when a process wants access to a resource. This would be equivalent to the arriving customer trying to get an open table. If there is an open table, or the semaphore is greater than zero, then he can take that resource and sit at the table. If there is no open table and the semaphore is zero, that process must wait until it becomes available. signal() is called when a process is done using a resource, or when the patron is finished with his meal.

• The following is an implementation of this counting semaphore (where the value can be greater than 1):

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• In this implementation, a process wanting to enter its critical section it has to acquire the binary semaphore which will then give it mutual exclusion until it signals that it is done.

• For example, we have semaphore s, and two processes, P1 and P2 that want to enter their critical sections at the same time. P1 first calls wait(s). The value of s is decremented to 0 and P1 enters its critical section. While P1 is in its critical section, P2 calls wait(s), but because the value of s is zero, it must wait until P1 finishes its critical section and executes signal(s).

• When P1 calls signal, the value of s is incremented to 1, and P2 can then proceed to execute in its critical section (after decrementing the semaphore again). Mutual exclusion is achieved because only one process can be in its critical section at any time.

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Example 22This example shows a simple data guard that can be used to protect data from being written by another task. The SWP instruction “holds the bus” until the transaction is complete.

loopMOV r1, =semaphoreMOV r2, #1SWP r3, r2, [r1] ; hold the bus until completeCMP r3, #1BEQ loop

The address pointed to by the semaphore either contains the value 0 or 1. When the semaphore equals 1, then the service in question is being used by another process. Theroutine will continue to loop around until the service is released by the other process—in other words, when the semaphore address location contains the value 0.

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ARM instructions by instruction class

1. Data Processing Instructions

2. Branch Instructions

3. Load-Store Instructions

4. Software Interrupt Instruction

5. Program Status Register Instructions

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Software Interrupt Instruction

• The software interrupt instruction is used for calls to the operating system and is often called a 'supervisor call'.

• It puts the processor into supervisor mode and begins executing instructions from address 0x08.

Binary encoding

Introduction

COND OPCODE 24-BIT (INTERPRETED) IMMEDIATE

31 28 27 24 23 0

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The 24-bit immediate field does not influence the operation of the instruction but may be interpreted by the system code. If the condition is passed the instruction enters supervisor mode using the standard ARM exception entry sequence. In detail, the processor actions are:1. Save the address of the instruction after the SWI in r14_svc.2. Save the CPSR in SPSR_svc. 3. Enter supervisor mode and disable IRQs (but not FIQs) by setting CPSR[4:0]

to 100112 and CPSR[7] tol.4. Set the PC to and begin executing the instructions there.

Binary encoding

COND OPCODE 24-BIT (INTERPRETED) IMMEDIATE

31 28 27 24 23 0

Description

To return to the instruction after the SWI the system routine must not only copy r14_svc back into the PC, but it must also restore the CPSR from SPSR_svc.

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Syntax: SWI{<cond>} SWI_number

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Example 23Here we have a simple example of an SWI call with SWI number 0x123456, used by ARM toolkits as a debugging SWI. Typically the SWI instruction is executed in user mode.

PRE cpsr = nzcVqift_USERpc = 0x00008000lr = 0x003fffff; lr = r14r0 = 0x12

0x00008000 SWI 0x123456

POST cpsr = nzcVqIft_SVCspsr = nzcVqift_USERpc = 0x00000008lr = 0x00008004r0 = 0x12

Since SWI instructions are used to call operating system routines, you need some form of parameter passing. This is achieved using registers. In this example, register r0 is used to pass the parameter 0x12. The return values are also passed back via registers.

Code called the SWI handler is required to process the SWI call. The handler obtains the SWI number using the address of the executed instruction, which is calculated from the link register lr.

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ARM instructions by instruction class

1. Data Processing Instructions

2. Branch Instructions

3. Load-Store Instructions

4. Software Interrupt Instruction

5. Program Status Register Instructions(MSR, MRS) (Self Study!!!) Refer Steve Furber

Byte organizations • Little-endian mode:- with the lowest-order byte residing in the low-

order bits of the word

• Big-endian mode: - the lowest-order byte stored in the highest bits

of the word

Byte organizations

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Thumb Mode

• Thumb is a 16-bit instruction set– Optimized for code density from C code– Improved performance form narrow memory– Subset of the functionality of the ARM instruction set

• Core has two execution states – ARM and Thumb– Switch between them using BX instruction

• Thumb has characteristic features:– Most Thumb instruction are executed unconditionally– Many Thumb data process instruction use a 2-address format– Thumb instruction formats are less regular than ARM

instruction formats, as a result of the dense encoding.

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Thumb has higher code density !

• Code density: it is define as the space taken up in memory by an executable program.

• On average, a Thumb implementation of the same code takes up around 30% less memory than the equivalent ARM implementation.

• Figure 4.1 shows the same divide code routine implemented in ARM and Thumb assembly code. Even though the Thumb implementation uses more instructions, the overall memory footprint is reduced. Code density was the main driving force for the Thumb instruction set.

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Thumb implementation uses more instructions, the overall memory footprint is reduced.

Code density was the main driving force for the Thumb instruction set. Because it was also designed as a compiler target, rather than for hand-written assembly code, we recommend that you write Thumb-targeted code in a high-level language like C or C++.

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Thumb Register Usage• In Thumb state, you do not have direct access to all registers.

• Only the low registers r0 to r7 are fully accessible.

• The higher registers r8 to r12 are only accessible with MOV, ADD, or CMP instructions.

• CMP and all the data processing instructions that operate on low registers update the condition flags in the cpsr.

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Thumb Instruction Set (1/3)

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Thumb Instruction Set (2/3)

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Thumb Instruction Set (3/3)

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Thumb Instruction Entry and Exit

T bit, bit 5 of CPSR If T = 1, the processor interprets the instruction stream as 16-bit Thumb

instruction If T = 0, the processor interprets if as standard ARM instructions

Thumb Entry ARM cores startup, after reset, execution ARM instructions Executing a branch and Exchange instruction (BX)

Set the T bit if the bottom bit of the specified register was set Switch the PC to the address given in the remainder of the register

Thumb Exit Executing a thumb BX instruction

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ARM-Thumb Interworking• ARM-Thumb interworking is the name given to the method of linking

ARM and Thumb code together for both assembly and C/C++.

• To call a Thumb routine from an ARM routine, the core has to change state. This state change is shown in the T bit of the cpsr.

• The BX and BLX branch instructions cause a switch between ARM and Thumb state while branching to a routine.

• The BX lr instruction returns from a routine, also with a state switch if necessary.

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• There are two versions of the BX or BLX instructions: an ARM instruction and a Thumb equivalent.

• The ARM BX instruction enters Thumb state only if bit 0 of the address in Rn is set to binary 1; otherwise it enters ARM state. The Thumb BX instruction does the same.

Syntax: BX RnBLX Rn | label

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Interworking Instructions

• Interworking is achieved using the Branch Exchange instructions– In Thumb state

BX Rn– In ARM state (on Thumb-aware cores only)

BX<condition> RnWhere Rn can be any registers (R0 to R15)

• The performs a branch to an absolute address in 4GB address space by copying Rn to the program counter

• Bit 0 of Rn specifies the state to change to

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Switching between States

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Example 24

;Start off in ARM stateCODE32ADR r0,Into_Thumb+1 ;generate branch target

;address & set bit 0;hence arrive Thumb

stateBX r0 ;branch exchange to Thumb…CODE16 ;assemble subsequent as Thumb

Into_Thumb …ADR r5,Back_to_ARM ;generate branch target to

;word-aligned address,;hence bit 0 is

cleared.BX r5 ;branch exchange to ARM…CODE32 ;assemble subsequent as ARM

Back_to_ARM …

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Summary

ARM data instructions

• MLA r0,rl,r2,r3 ,r0=r1 x r2 + r3

ADD ADC SUB SBC RSB RSC MUL MLA

AddAdd with carrySubtractSubtract with carryReverse subtract ,RSB r0,r1,r2, r0=r2 – r1Reverse subtract with carryMultiplyMultiply and accumulate

ARM data instructions

• BIC r0,r1,r2 sets r0 to r1 and not r2 - uses the second source operand as a mask, a bit in

mask is 1, the corresponding bit in first source operand is cleared

ANDORREORBIC

Bit-wise andBit-wise orBit-wise exclusive-orBit clear

ARM data instructionsLSL LSR ASL ASR ROR RRX

Logical shift left (zero fill) Logical shift right (zero fill) Arithmetic shift left Arithmetic shift right, copies the sign bit Rotate right Rotate right extended with C, performs a 33-bit rotate

ARM comparison instructions

• only set the values of the NZCV bits

CMPCMN

TSTTEQ

CompareNegated compare, uses an addition to set the status bits Bit-wise test, a bit-wise AND Bit-wise negated test, an exclusive-or

ARM move instructions

MOV MVN

MoveMOV r0,r1 ; r0=r1Move negated Mvn r0,r1 ; r0=not(r1)

ARM load-store instructions

LDRSTRLDRHSTRHLDRSHLDRBSTRBADR

LoadStoreLoad half-wordStore half-wordLoad half-word signedLoad byteStore byteSet register to address

C Assignments in ARM Instructions

• x = (a + b) - c;• using r0 for a, r1 for b, r2 for c, and r3 for x. • registers for indirect addressing. Indirect r4• load values of a, b, and c into registers • store value of x back to memory

C Assignments in ARM Instructions x = (a + b) - c;

ADR r4,a ; get address for aLDR r0,[r4] ; get value of aADR r4,b ; get address for b, using r4LDR r1,[r4] ; load value of bADD r3,r0,r1 ; set result for x to a + bADR r4,c ; get address for cLDR r2,[r4] ; get value of cSUB r3,r3,r2 ; complete computation of xADR r4,x ; get address for xSTR r3,[r4] ; store x at proper location

C Assignments in ARM Instructions

• y = a*(b + c);• using r0 for both a and b, r1 for c, and r2 for y• use r4 to store addresses for indirect

addressing

C Assignments in ARM Instructions y = a*(b + c);

ADR r4,b ; get address for bLDR r0,[r4] ;get value of bADR r4,c ; get address for cLDR r1,[r4] ; get value of cADD r2,r0,r1 ; compute partial result of y=b+cADR r4,a ; get address for aLDR r0,[r4] ; get value of aMUL r2,r2,r0 ; compute final value of y=a*(b+c)ADR r4,y ; get address for ySTR r2,[r4] ; store value of y at proper location

C Assignments in ARM Instructions

• z = (a « 2) | (b & 15);• using r0 for a and z, r1 for b, • r4 for addresses

C Assignments in ARM Instructions z = (a « 2) | (b & 15);

ADR r4,a ; get address for aLDR r0,[r4] ; get value of aMOV r0,r0,LSL 2 ; perform shift (a « 2)ADR r4,b ; get address for bLDR r1,[r4] ; get value of bAND r1,r1,#15 ; perform logical AND (b & 15)ORR r1,r0,r1 ; compute final value of zADR r4,z ; get address for zSTR r1,[r4] ; store value of z