ARM7-ARCHITECTURE

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ARM7-ARCHITECTURE

  1. 1. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.com UNIT-II ARM 7 MicrocontrollerINTRODUCTION:The ARM was originally developed at Acorn Computers Limited of Cambridge , England,between 1983 and 1985. It was the first RISC microprocessor developed for commercial use andhas some significant differences from subsequent RISC architectures. In 1990 ARM Limitedwas established as a separate company specifically to widen the exploitation of ARM technologyand it is established as a market-leader for low-power and cost-sensitive embeddedapplications. The ARM is supported by a toolkit which includes an instruction set emulator forhardware modelling and software testing and benchmarking, an assembler, C and C++compilers, a linker and a symbolic debugger.The 16-bit CISC microprocessors that were available in 1983 were slower than standard memoryparts. They also had instructions that took many clock cycles to complete (in some cases, manyhundreds of clock cycles), giving them very long interrupt latencies.As a result of thesefrustrations with the commercial microprocessor offerings, the design of a proprietarymicroprocessor was considered and ARM chip was designed.ARM 7TDMI-S Processor : The ARM7TDMI-S processor is a member of the ARM family ofgeneral-purpose 32-bit microprocessors. The ARM family offers high performance for verylow-power consumption and gate count. The ARM7TDMI-S processor has a Von Neumannarchitecture, with a single 32-bit data bus carrying both instructions and data. Only load, store,and swap instructions can access data from memory. The ARM7TDMI-S processor uses a threestage pipeline to increase the speed of the flow of instructions to the processor. This enablesseveral operations to take place simultaneously, and the processing, and memory systems tooperate continuously. In the three-stage pipeline the instructions are executed in three stages.The three stage pipelined architecture of the ARM7 processor is shown in the above figure.1
  2. 2. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comARM7TDMIS stands forT: THUMB ;D for on-chip Debug support, enabling the processor to halt in response to a debug request,M: enhanced Multiplier, yield a full 64-bit result, high performanceI: Embedded ICE hardware (In Circuit emulator)S : SynthesizableFEATURES OF ARM PROCESSORSThe ARM processors are based on RISC architectures and this architecture has provided smallimplementations, and very low power consumption. Implementation size, performance, and verylow power consumption remain the key features in the development of the ARM devices.The typical RISC architectural features of ARM are : A large uniform register file A load/store architecture, where data-processing operations only operate on register contents, not directly on memory contents Simple addressing modes, with all load/store addresses being determined from register contents and instruction fields only uniform and fixed-length instruction fields, to simplify instruction decode. Control over both the Arithmetic Logic Unit (ALU) and shifter in most data-processing instructions to maximize the use of an ALU and a shifter Auto-increment and auto-decrement addressing modes to optimize program loops Load and Store Multiple instructions to maximize data throughput Conditional execution of almost all instructions to maximize execution throughput.There are three basic instruction sets for ARM. A 32- bit ARM instruction set A 16 –bit Thumb instruction set and The 8-bit Java Byte code used in Jazelle stateThe Thumb instruction set is a subset of the most commonly used32-bit ARMinstructions.Thumb instructions operate with the standard ARM register configurations,enabling excellent interoperability between ARM and Thumb states.This Thumb state is nearly2
  3. 3. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.com65% of the ARM code and can provide 160%of the performance of ARM code when workingona 16-bit memory system. This Thumb mode is used in embedded systems where memoryresources are limited. The Jazelle mode is used in ARM9 processor to work with 8-bit Javacode.ARCHITECTURE OF ARM PROCESSORS:The ARM 7 processor is based on Von Neman model with a single bus for both data andinstructions..( The ARM9 uses Harvard model).Though this will decrease the performance ofARM, it is overcome by the pipe line concept. ARM uses the Advanced Microcontroller BusArchitecture (AMBA) bus architecture. This AMBA include two system buses: the AMBAHigh-Speed Bus (AHB) or the Advanced System Bus (ASB), and the Advanced Peripheral Bus(APB).The ARM processor consists of Arithmetic Logic Unit (32-bit) One Booth multiplier(32-bit) One Barrel shifter One Control unit Register file of 37 registers each of 32 bits.In addition to this the ARM also consists of a Program status register of 32 bits, Somespecial registers like the instruction register, memory data read and write register andmemory address register ,one Priority encoder which is used in the multiple load andstore instruction to indicate which register in the register file to be loaded or stored andMultiplexers etc.3
  4. 4. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comARM Registers : ARM has a total of 37 registers .In which - 31 are general-purpose registersof 32-bits, and six status registers .But all these registers are not seen at once. The processorstate and operating mode decide which registers are available to the programmer. At any time,among the 31 general purpose registers only 16 registers are available to the user. The remaining15 registers are used to speed up exception processing. there are two program status registers:CPSR and SPSR (the current and saved program status registers, respectivelyIn ARM state the registers r0 to r13 are orthogonal—any instruction that you can apply to r0 youcan equally well apply to any of the other registers.The main bank of 16 registers is used by all unprivileged code. These are the User moderegisters. User mode is different from all other modes as it is unprivileged. In addition to thisregister bank ,there is also one 32-bit Current Program status Register(CPSR)4
  5. 5. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comIn the 15 registers ,the r13 acts as a stack pointer register and r14 acts as a link register and r15acts as a program counter register.Register r13 is the sp register ,and it is used to store the address of the stack top. R13 is used bythe PUSH and POP instructions in T variants, and by the SRS and RFE instructions fromARMv6.Register 14 is the Link Register (LR). This register holds the address of the next instruction aftera Branch and Link (BL or BLX) instruction, which is the instruction used to make a subroutinecall. It is also used for return address information on entry to exception modes. At all other times,R14 can be used as a general-purpose register.Register 15 is the Program Counter (PC). It can be used in most instructions as a pointer to theinstruction which is two instructions after the instruction being executed.The remaining 13 registers have no special hardware purpose.CPSR : The ARM core uses the CPSR register to monitor and control internal operations. TheCPSR is a dedicated 32-bit register and resides in the register file. The CPSR is divided intofour fields, each of 8 bits wide : flags, status, extension, and control. The extension and statusfields are reserved for future use. The control field contains the processor mode, state, andinterrupt mask bits. The flags field contains the condition flags. The 32-bit CPSR register isshown below.5
  6. 6. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comProcessor Modes: There are seven processor modes .Six privileged modes abort, fast interruptrequest, interrupt request, supervisor, system, and undefined and one non-privileged modecalled user mode.The processor enters abort mode when there is a failed attempt to access memory. Fast interruptrequest and interrupt request modes correspond to the two interrupt levels available on the ARMprocessor. Supervisor mode is the mode that the processor is in after reset and is generally themode that an operating system kernel operates in. System mode is a special version of user modethat allows full read-write access to the CPSR. Undefined mode is used when the processorencounters an instruction that is undefined or not supported by the implementation. User mode isused for programs and applications.Banked Registers : Out of the 32 registers , 20 registers are hidden from a program at differenttimes. These registers are called banked registers and are identified by the shading in thediagram. They are available only when the processor is in a particular mode; for example, abortmode has banked registers r13_abt , r14_abt and spsr _abt. Banked registers of a particularmode are denoted by an underline character post-fixed to the mode mnemonic or _mode.When the T bit is 1, then the processor is in Thumb state. To change states the core executes aspecialized branch instruction and when T= 0 the processor is in ARM state and executes ARMinstructions. There are two interrupt request levels available on the ARM processor core—interrupt request (IRQ) and fast interrupt request (FIQ).V, C , Z , N are the Condition flags .V (oVerflow) : Set if the result causes a signed overflowC (Carry) : Is set when the result causes an unsigned carryZ (Zero) : This bit is set when the result after an arithmetic operation is zero, frequently used to indicate equalityN (Negative) : This bit is set when the bit 31 of the result is a binary 1.PIPE LINE : Pipeline is the mechanism used by the RISC processor to execute instructions atan increased speed. This pipeline speeds up execution by fetching the next instruction whileother instructions are being decoded and executed. During the execution of an instruction ,theprocessor Fetches the instruction .It means loads an instruction from memory.And decodes the6
  7. 7. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.cominstruction i.e identifies the instruction to be executed and finally Executes the instruction andwrites the result back to a register.The ARM7 processor has a three stage pipelining architecture namely Fetch , Decode andExecute.And the ARM 9 has five stage Pipe line architecture.The three stage pipelining isexplained as below.To explain the pipelining ,let us consider that there are three instructions Compare, Subtract andAdd.The ARM7 processor fetches the first instruction CMP in the first cycle and during thesecond cycle it decodes the CMP instruction and at the same time it will fetch the SUBinstruction. During the third cycle it executes the CMP instruction , while decoding the SUBinstruction and also at the same time will fetch the third instruction ADD. This will improve thespeed of operation. This leads to the concept of parallel processing .This pipeline example isshown in the following diagram.7
  8. 8. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comAs the pipeline length increases, the amount of work done at each stage is reduced, which allowsthe processor to attain a higher operating frequency. This in turn increases the performance. Oneimportant feature of this pipeline is the execution of a branch instruction or branching by thedirect modification of the PC causes the ARM core to flush its pipeline.Exceptions, Interrupts, and the Vector Table :Exceptions are generated by internal and external sources to cause the ARM processor to handlean event, such as an externally generated interrupt or an attempt to execute an Undefinedinstruction. The processor state just before handling the exception is normally preserved so thatthe original program can be resumed after the completion of the exception routine. More thanone exception can arise at the same time.ARM exceptions may be considered in three groups1. Exceptions generated as the direct effect of executing an instruction.Software interrupts,undefined instructions (including coprocessor instructions where the requested coprocessor isabsent) and prefetch aborts (instructions that are invalid due to a memory fault occurring duringfetch) come under this group.2. Exceptions generated as a side-effect of an instruction.Data aborts (a memory fault during aload or store data access) are in this group.3. Exceptions generated externally, unrelated to the instruction flow.Reset, IRQ and FIQ are inthis group.The ARM architecture supports seven types of exceptions.i.Resetii.Undefined Instructioniii.Software Interrupt(SWI)iv. Pre-fetch abort(Instruction Fetch memory fault)v.Data abort (Data access memory fault)vi. IRQ(normal Interrupt)vii. FIQ (Fast Interrupt request).When an Exception occurs , the processor performs the following sequence of actions:• It changes to the operating mode corresponding to the particular exception.• It saves the address of the instruction following the exception entry instruction in r14 of the new mode.8
  9. 9. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.com• It saves the old value of the CPSR in the SPSR of the new mode.• It disables IRQs by setting bit 7 of the CPSR and, if the exception is a fast interrupt, disables further fast interrupts by setting bit 6 of the CPSR.• It forces the PC to begin executing at the relevant vector address Excdption / Interrupt Name Address High AddressReset RESET 0X00000000 0Xffff0000Undefined Instruction UNDEF 0X00000004 0Xffff0004Software Interrupt SWI 0X00000008 0Xffff0008Pre-fetch Abort PABT 0X0000000C 0Xffff000cData Abort DABT 0X00000010 0Xffff0010Reserved --- 0X00000014 0Xffff0014Interrupt Request IRQ 0X00000018 0Xffff0018Fast Interrupt Request FIQ 0X0000001C 0Xffff001cThe exception Vector table shown above gives the address of the subroutine program to beexecuted when the exception or interrupt occurs. Each vector table entry contains a form ofbranch instruction pointing to the start of a specific routine.Reset vector is the location of the first instruction executed by the processor when power isapplied. This instruction branches to the initialization code.Undefined instruction vector is used when the processor cannot decode an instruction.Software interrupt vector is called when you execute a SWI instruction. The SWI instruction isfrequently used as the mechanism to invoke an operating system routine.Pre-fetch abort vector occurs when the processor attempts to fetch an instruction from an addresswithout the correct access permissions. The actual abort occurs in the decode stage.Data abort vector is similar to a prefetch abort but is raised when an instruction attempts toaccess data memory without the correct access permissions.Interrupt request vector is used by external hardware to interrupt the normal execution flow ofthe processor. It can only be raised if IRQs are not masked in the CPSR.ARM Processor FamiliesThere are various ARM processors available in the market for different application .These aregrouped into different families based on the core .These families are based on the ARM7,9
  10. 10. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comARM9, ARM10, and ARM11 cores. The numbers 7, 9, 10, and 11 indicate different coredesigns. The ascending number indicates an increase in performance and sophistication.Though ARM 8 was introduced during 1996, it is no more available in the market. Thefollowing table gives a brief comparison of their performance and available resources.The ARM7 core has a Von Neumann–style architecture, where both data and instructions use thesame bus. The core has a three-stage pipeline and executes the architecture ARMv4T instructionset. The ARM7TDMI was introduced in 1995 by ARM. It is currently a very popular core and isused in many 32-bit embedded processors.The ARM9 family was released in 1997. It has five stage pipeline architecture .Hence , theARM9 processor can run at higher clock frequencies than the ARM7 family. The extra stagesimprove the overall performance of the processor. The memory system has been redesigned tofollow the Harvard architecture, with separate data and instruction .buses. The first processor inthe ARM9 family was the ARM920T, which includes a separate D + I cache and an MMU. Thisprocessor can be used by operating systems requiring virtual memory support. ARM922T is avariation on the ARM920T but with half the D +I cache size.The latest core in the ARM9 product line is the ARM926EJ-S synthesizable processor core,announced in 2000. It is designed for use in small portable Java-enabled devices such as 3Gphones and personal digital assistants (PDAs).The ARM10 was released in 1999 . It extends the ARM9 pipeline to six stages. It also supportsan optional vector floating-point (VFP) unit, which adds a seventh stage to the ARM10 pipeline.The VFP significantly increases floating-point performance and is compliant with the IEEE754.1985 floating-point standard.The ARM1136J-S is the ARM11 processor released in the year 2003 and it is designed for highperformance and power efficient applications. ARM1136J-S was the first processorimplementation to execute architecture ARMv6 instructions. It incorporates an eight-stagepipeline with separate load store and arithmetic pipelines.A brief comparison of different ARM families is presented below.10
  11. 11. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.com ARM Year of Architecture Pipeline Operational Multiplier MIPS Family Release Frequency ARM7 1995 Von Neumann 3 stage 80 M.Hz 8x32 0.97 ARM9 1997 Harvard 5 stage 150M.Hz 8x32 1.1 ARM10 1999 Harvard 6 stage 260M.Hz 16x32 1.3 ARM11 2003 Harvard 8 stage 335M.Hz 16x32 1.2ARM INSTRUCTION SETARM instructions process data held in registers and only access memory with load and storeinstructions. ARM instructions commonly take two or three operands.For example ,the ADD instruction adds the two values stored in registers r1 and r2 (the sourceregisters). It stores the result to register r3 (the destination register). ADD r3, r1, r2ARM instructions are classified into data processing instructions, branch instructions, load-storeinstructions, software interrupt instruction, and program status register instructions.Data Processing Instructions :The data processing instructions manipulate data within registers. They are move instructions,Arithmetic instructions, logical instructions, comparison instructions, and multiply instructions.Most data processing instructions can process one of their operands using the barrel shifter.Data processing instructions are processed within the arithmetic logic unit (ALU). A unique andpowerful feature of the ARM processor is the ability to shift the 32-bit binary pattern in one ofthe source registers left or right by a specific number of positions before it enters the ALU. Thisshift increases the power and flexibility of many data processing operations.There are data processing instructions that do not use the barrel shift, for example, the MUL(multiply), CLZ (count leading zeros), and QADD (signed saturated 32-bit add) instructions.i.Move Instructions : Move instruction copies R into a destination register Rd, where R is aregister or immediate value. This instruction is useful for setting initial values and transferringdata between registers.11
  12. 12. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comExample1 : PRE r5 = 5 r7 = 8 MOV r7, r5 ; POST r5 = 5 r7 = 5The MOV instruction takes the contents of register r5 and copies them into register r7.Example 2: MOVS r0, r1, LSL #1 MOVS instruction shifts register r1 left by one bit Arithmetic Instructions : The arithmetic instructions implement addition and subtraction of32-bit signed and unsigned values. the various addition and subtraction instructions are given intable below.SUB r0, r1, r2 ; This subtract instruction subtracts a value stored in register r2 from a value stored in register r1. The result is stored in register r0.RSB r0, r1, #0 ; This reverse subtract instruction (RSB) subtracts r1 from the constant value #0, writing. the result to r0. You can use this instruction to negate numbers.SUBS r1, r1, #1 ; The SUBS instruction is useful for decrementing loop counters. In this example we subtract the immediate value one from the value one stored in register r1. The result value zero is written to register r1.Logical Instructions : These Logical instructions perform bitwise logical operations on the twosource registers.12
  13. 13. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comBIC r0, r1, r2 ; BIC, carries out a logical bit clear. register r2 contains a binary pattern whereevery binary 1 in r2 clears a corresponding bit location in register r1. This instruction isparticularly useful when clearing status bits and is frequently used to change interrupt masks inthe cpsr.Comparison Instructions : The comparison instructions are used to compare or test aregister with a 32-bit value. This instruction affects only CPSR register flags.Branch Instructions: A branch instruction changes the normal flow of execution of a mainprogram or is used to call a subroutine routine. This type of instruction allows programs to havesubroutines, if-then-else structures, and loops. The change of execution flow forces the programcounter pc to point to a new address.Example 1: B forward ; (unconditional branch to forward) ADD r1, r2, #4 ; ADD r0, r6, #2 ; ADD r3, r7, #4 ; forward SUB r1, r2, #4 ;Similarly Backward branch :13
  14. 14. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.com backward : ADD r1, r2, #4 ; SUB r1, r2, #4 ; ADD r4, r6, r7 ; B backward ; branch to the target backward.The branch with link, or BL, instruction is similar to the B instruction but overwrites thelink register lr with a return address. It performs a subroutine call. BL subroutine ; branch to subroutine CMP r1, #5 ; compare r1 with 5 MOVEQ r1, #0 ; if (r1==5) then r1 = 0 : Subroutine MOV pc, lr ; return by moving pc = lrThe Branch Exchange (BX) and Branch Exchange with Link (BLX) are the third type of branch instruction.The BX instruction uses an absolute address stored in register Rm. It is primarily used to branch to andfrom Thumb code. The T bit in the cpsr is updated by the least significant bit of the branch register.Similarly the BLX instruction updates the T bit of the cpsr with the least significant bit and additionallysets the link register with the return address.The details of the branch instructions are given in the table above.Load-Store Instructions : Load-store instructions transfer data between memory and processorregisters. There are three types of load-store instructions: Single-register transfer Multiple-register transfer, and14
  15. 15. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.com Swap.Single-Register Transfer : These instructions are used for moving a single data item in and out of aregister. The data types supported are signed and unsigned words (32-bit), half-words (16-bit), andbytes. Ex1: STR r0, [r1] ; = STR r0, [r1, #0] ; store the contents of register r0 to thememory address pointed to by register r1.Ex2 : LDR r0, [r1] ; = LDR r0, [r1, #0] ; load register r0 with the contents of the memory address pointed to by register r1.Multiple-Register Transfer : Load-store multiple instructions can transfer multiple registersbetween memory and the processor in a single instruction. The transfer occurs from a baseaddress register Rn pointing into memory. Multiple-register transfer instructions are moreefficient than single-register transfers for moving blocks of data around memory and saving andrestoring context and stacks.Load-store multiple instructions can increase interrupt latency. ARM implementations do notusually interrupt instructions while they are executing. For example, on an ARM7 a loadmultiple instruction takes 2 + N.t cycles, where N is the number of registers to load and t is thenumber of cycles required for each sequential access to memory. If an interrupt has been raised,then it has no effect until the load-store multiple instruction is complete.Example 1: LDMIA r0!, {r1-r3} ; In this example, register r0 is the base register Rn and isfollowed by !, indicating that the register is updated after the instruction is executed. In this casethe range is from register r1 to r3.Example 2 : LDMIB : load multiple and increment before Ex 3: LDMIB r0!, {r1-r3} ; Ex 4 : LDMDA r0!, {r1-r3}15
  16. 16. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comStack Operations : The ARM architecture uses the load-store multiple instructions to carry outstack operations. The pop operation (removing data from a stack) uses a load multipleinstruction; similarly, the push operation (placing data onto the stack) uses a store multipleinstruction.A stack is either ascending (A) or descending (D). Ascending stacks grow towards highermemory addresses; in contrast, descending stacks which grow towards lower memory addresses.When a full stack (F)is used , the stack pointer sp points to an address that is the last used or fulllocation (i.e., sp points to the last item on the stack). In contrast, if an empty stack (E) is used ,the sp points to an address that is the first unused or empty location (i.e., it points after the lastitem on the stack).Example1 : The STMFD instruction pushes registers onto the stack, updating the sp. STMFD sp! , {r1,r4}; Store Multiple Full Descending Stack PRE r1 = 0x00000002 r4 = 0x00000003 sp = 0x00080014 POST r1 = 0x00000002 r4 = 0x00000003 sp = 0x0008000c.The stack operation is shown by the following diagram.Example2: The STMED instruction pushes the registers onto the stack but updates register sp topoint to the next empty location as shown in the below diagram..16
  17. 17. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.com PRE r1 = 0x00000002 r4 = 0x00000003 sp = 0x00080010 STMED sp! , {r1,r4} ; Store Multiple Empty Descending Stack POST r1 = 0x00000002 r4 = 0x00000003 sp = 0x00080008This operation is explained in the following figure.Swap Instruction :The Swap instruction is a special case of a load-store instruction. It swaps (Similar to exchange)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 fromreading or writing to that location until it completes.Swap cannot be interrupted by any otherinstruction or any other bus access. So, the system ―holds the bus‖ until the transaction iscomplete.Ex 1: SWP : Swap a word between memory and a register tmp = mem32[Rn] mem32[Rn] =Rm Rd = tmpEx2 : SWPB Swap a byte between memory and a register tmp = mem8[Rn] mem8[Rn] =Rm17
  18. 18. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.com Rd = tmp. Ex 3: SWP r0, r1, [r2] ; The swap instruction loads a word from memory into register r0 and overwrites the memory with register r1.Software Interrupt Instruction : A software interrupt instruction (SWI) is used to generate asoftware interrupt exception, which can be used to call operating system routines.When theprocessor executes an SWI instruction, it sets the program counter pc to the offset 0x8 in thevector table. The instruction also forces the processor mode to SVC, which allows an operatingsystem routine to be called in a privileged mode. Each SWI instruction has an associated SWInumber, which is used to represent a particular function call or feature.Ex: 0x00008000 SWI 0x123456Here 0x123456, is the SWI number used by ARM toolkits as a debugging SWI. Typicallythe SWI instruction is executed in user mode.Program Status Register Instructions : There are two instructions available to directly controla program status register (PSR). The MRS instruction transfers the contents of either the CPSRor SPSR into a register.Similarly the MSR instruction transfers the contents of a register intothe CPSR or SPSR .These instructions together are used to read and write the CPSR andSPSR. MRS : copy program status register to a general-purpose register , Rd= PSR MSR : move a general-purpose register to a program status register, PSR[field]=Rm MSR : move an immediate value to a program status register, PSR[field]=immediate Ex : MRS r1, CPSR BIC r1, r1, #0x80 ; 0b01000000 MSR CPSR_C, r1.The MSR first copies the CPSR into register r1. The BIC instruction clears bit 7 of r1. Registerr1 is then copied back into the CPSR, which enables IRQ interrupts. Here the code preserves allthe other settings in the CPSR intact and only modifies the I bit in the control field.18
  19. 19. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comLoading Constants : In ARM instruction set there are no instructions to move the 32-bitconstant into a register. Since ARM instructions are 32 bits in size, they obviously cannotspecify a general 32-bit constant. To overcome this problem .two pseudo instructions areprovided to move a 32-bit value into a register. LDR : load constant pseudo instruction Rd= 32-bit constant. ADR : load address pseudo instruction Rd=32-bit relative address.The first pseudo instruction writes a 32-bit constant to a register using whatever instructions areavailable.The second pseudo instruction writes a relative address into a register, which will be encodedusing a PC -relative expression.Example 2: LDR r0, [pc, #constant_number-8-{PC}] constant _number DCD 0xff00ffff.Here the LDR instruction loads a 32-bit constant 0xff00ffff into register r0.Example 3: The same constant can be loaded into the register r0 using the MVN instruction also. MVN r0, #0x00ff0000 After execution r0 = 0xff00ffff.Introduction to Thumb instruction set : Thumb encodes a subset of the 32-bit ARMinstructions into a 16-bit instruction set space. Since Thumb has higher performance than ARMon a processor with a 16-bit data bus, but lower performance than ARM on a 32-bit data bus, useThumb for memory-constrained systems. Thumb has higher code density—the space taken up inmemory by an executable program—than ARM. For memory-constrained embedded systems,for example, mobile phones and PDAs, code density is very important. Cost pressures also limitmemory size, width, and speed.Thumb execution is flagged by the T bit (bit [5] ) in the CPSR. A Thumb implementation of thesame code takes up around 30% less memory than the equivalent ARM implementation. Eventhough the Thumb implementation uses more instructions ; the overall memory footprint isreduced. Code density was the main driving force for the Thumb instruction set. Because it wasalso designed as a compiler target, rather than for hand-written assembly code. Below exampleexplains the difference between ARM and Thumb code19
  20. 20. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comFrom the above example it is clear that the Thumb code is more denser than the ARM code.Exceptions generated during Thumb execution switch to ARM execution before executing theexception handler . The state of the T bit is preserved in the SPSR, and the LR of the exceptionmode is set so that the normal return instruction performs correctly, regardless of whether theexception occurred during ARM or Thumb execution.In Thumb state, all the registers can not be accessed . Only the low registers r0 to r7 can beaccessed. The higher registers r8 to r12 are only accessible with MOV, ADD, or CMPinstructions. CMP and all the data processing instructions that operate on low registers updatethe condition flags in the CPSRThe list of registers and their accessibility in Thumb mode are shown in the following table.. S.No Registers Access 1 r0 – r7 Fully accessible 2 r8 – r12 Only accessible by MOV ,ADD &CMP 3 r13SP Limited accessibility 4 r14 lr Limited accessibility 5 r15 PC Limited accessibility 6 CPSR Only indirect access 7 SPSR No access20
  21. 21. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.comForm the above discussion, it is clear that there are no MSR and MRS equivalent Thumbinstructions. To alter the CPSR or SPSR , one must switch into ARM state to use MSR andMRS. Similarly, there are no coprocessor instructions in Thumb state. You need to be in ARMstate to access the coprocessor for configuring cache and memory management.ARM-Thumb interworking is the method of linking ARM and Thumb code together for bothassembly and C/C++. It handles the transition between the two states. To call a Thumb routinefrom an ARM routine, the core has to change state. This is done with the T bit of CPSR . TheBX and BLX branch instructions cause a switch between ARM and Thumb state while branchingto a routine. The BX lr instruction returns from a routine, also with a state switch if necessary.The data processing instructions manipulate data within registers. They include moveinstructions, arithmetic instructions, shifts, logical instructions, comparison instructions, andmultiply instructions. The Thumb data processing instructions are a subset of the ARM dataprocessing instructions.Exs : ADC : add two 32-bit values and carry Rd = Rd + Rm + C flag ADD : add two 32-bit values Rd = Rn + immediate Rd = Rd + immediate Rd = Rd + Rm AND : logical bitwise AND of two 32-bit values Rd = Rd & Rm ASR : arithmetic shift right Rd = Rm_immediate, C flag= Rm[immediate − 1] Rd = Rd_Rs, C flag = Rd[Rs - 1] BIC : logical bit clear (AND NOT) of two 32-bit Rd = Rd AND NOT(Rm)values CMN : compare negative two 32-bit values Rn + Rm sets flags CMP : compare two 32-bit integers Rn−immediate sets flags Rn−Rm sets flags EOR : logical exclusive OR of two 32-bit values Rd = Rd EOR Rm LSL : logical shift left Rd = Rm_ immediate, C flag= Rm[32 − immediate] Rd = Rd_Rs, C flag = Rd[32 − Rs] LSR : logical shift right Rd = Rm_ immediate, C flag = Rd [immediate − 1] Rd = Rd_ Rs, C flag = Rd[Rs − 1] MOV : move a 32-bit value into a register Rd = immediate21
  22. 22. Dr.Y.Narasimha MurthyPh.D yayavaram@yahoo.com Rd = Rn Rd = Rm MUL : multiply two 32-bit values Rd = (Rm * Rd)[31:0] MVN : move the logical NOT of a 32-bit value into a register Rd = NOT(Rm) NEG : negate a 32-bit value Rd = 0 − Rm ORR : logical bitwise OR of two 32-bit values Rd = Rd OR Rm ROR : rotate right a 32-bit value Rd = Rd RIGHT_ROTATE Rs, C flag= Rd[Rs−1] SBC : subtract with carry a 32-bit value Rd = Rd − Rm − NOT(C flag) SUB : subtract two 32-bit values Rd = Rn − immediate Rd = Rd − immediate Rd = Rn − Rm sp = sp − (immediate_2) TST : test bits of a 32-bit value Rn AND Rm sets flagsNote : Thumb deviates from the ARM style in that the barrel shift operations (ASR, LSL, LSR, and ROR) are separate instructions.Thankful to the following people for their invaluable information .References : 1. ARM System Developer’s Guide Designing and Optimizing System Software – Andrew N. Sloss , Dominic Symes and Chris Wright. 2. ARM-System on-Chip Architecture – Steve Furber. 3. ARM Architecture Reference Manual Copyright © 1996-1998, 2000, 2004, 2005 ARM Limited.22

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