ARM Microprocessor “MIPS for the Masses”

Brief History ARM (Advanced Risc Machine) Microprocessor was based on the Berkeley/Stanford Risc concept Originally called Acorn Risc Machine because developed by Acorn Computer in 1985 Financial troubles initially plagued the Acorn company but the ARM was rejuvenated by Apple, VLSI technology, and Nippon Investment and Finance

Basics Only 25 basic instruction types 3 stage instruction pipeline All instructions are one word long All instructions are predicated

Basics (Contd.) All data processing instructions operate on registers only All data processing instructions can use the barrel shifter (an interesting and unique feature) to shift or rotate an operand Can operate in either big or little endian mode

Pipeline 1 2 3 4 3 stage: fetch; decode; execute Clock Cycle Fetch Area Decode Area Execution Area 1 Instruction 1 Empty 2 Instruction 2 3 Instruction 3 4 Instruction 4

Memory Arm is a 32 bit microprocessor with one word being 32 bits long Memory is byte-oriented; each byte of memory has its own unique address Must use an address divisible by 4 to access a word though

Memory (contd.) ARM has a 26 bit wide addressing range which allows 64 mb of memory to be directly addressed Memory Access: Register Indirect Addressing Pre and Post Indexed Addressing PC Relative Addressing Byte and Word Addressing

Registers 32 bit registers: 13 general purpose registers, R0 to R12 R13 generally used as a Stack register R14 as the Link register R15 is the Program Counter and Status register

Program Counter (PC) Program Counter is in bits 2 – 25 of R15 After fetching an instruction, PC is incremented to next word PC only needs to be 24 bits in length, though it can address a 26 bit address space – instructions must start at a word boundary thus 2 least significant bits must be zero

Status Register Status register bits indicate either processor mode, fast interrupt mode, normal interrupt mode, and also the overflow, carry, zero, and negative flags

Condition Four most significant bits indicate one of sixteen possible conditions for an instruction: EQ (Equal) 0000; NE (Not Equal) 0001; CS (Carry Set) 0010; CC (Carry Clear) 0011; MI (Minus) 0100; PL (Plus) 0101; VS (Overflow Set) 0110; VC (Overflow Clear) 0111; HI (Higher) 1000; LS (Lower or Same) 1001; GE (Greater or Equal) 1010; LT (Less Than) 1011; GT (Greater Than) 1100; LE (Less than or Equal) 1101; AL (Always) 1110; NV (Never) 1111 Given the condition bits, an instruction will either be executed or ignored depending on status bits

Branch Branch instruction can transfer program execution by loading a new value into the PC. Branch with link is same except address of next instruction is saved in R14, the link register – allows a single subroutine to be called

Stacks Stacks are implemented using LDM (Load Multiple Register) and STM (Store Multiple Register) instructions and FD, ED, EA, and FA can be added to represent full, descending; empty, descending; empty, ascending; and full, ascending, respectively – Used for nested or recursive subroutines

Interrupts Interrupts can be handled either by IRQ or by FIQ pins IRQ mode provides hidden registers R13_irq and R14_irq so that R13 and R14 vales will be unaffected when an external device interrupts normal processing FIQ is fast interrupt mode and registers R8_fiq - R14_fiq protect user mode registers R8 – R14

Software Interrupt Use of Software Interrupt instruction (SWI) causes ARM to go into supervisor mode with private registers R13_svc and R14_svc as extras to allow OS kernel to protect the stack and link registers

Instruction types 18 data processing instructions of type: <opcode> <dest. reg.> <op1> <op2> ADC Add with Carry; ADD Add; AND Bitwise logical AND; BIC Bit Clear CMN Compare Negated; CMP Compare; EOR Exclusive OR; MOV Move; MVN Move Not; ORR Bitwise logical OR; RSB Reverse Subtract; RSC Reverse Subtract with Carry; SBC Subtract with Carry; SUB Subtract; TEQ Test Equivalence; TST Test and Mask

Instruction types (contd.) 2 memory instructions, LDR and STR of type: <opcode> <op1> <op2> 2 branch instructions, B and BL of type: <opcode> <branch address> SWI <number> is Software Interrupt instruction --number represents some OS function

Immediate Addressing Direct Addressing Mode is not possible with the original ARM microprocessor – after setting aside bits for the opcode and operands, only 12 bits remain for an immediate value to be used This is not enough, but it simulates 32 bits in most cases by splitting the 12 bit field into an 8 bit data field and a 4 bit shift field

Immediate Addressing (contd.) 4 bit field can shift an 8 bit data field into any one of 16 possible positions If necessary to use a complete 32 bit word, then break it up into four groups of 8 bits and use shift and add instructions to reassemble it

Shift instructions Shift instruction fields are 5 bits, so shifts can accurately place in up to all 32 positions Shift instructions: LSL logical shift left, ASL Arithmetic shift left, LSR Logical shift right, ASR Arithmetic shift right, ROR Rotate right, RRX Rotate right with extend

Barrel Shifter (unique feature) Included in data path for any of the 18 data processing instructions is the barrel shifter, which allows a shift or rotate instruction (on 2nd operand) to be appended to these instructions without needing another cycle Eg. ADD R3, R1, R2, LSL#4 which shifts bits in R2 four places to the left and then adds to R1 and then places it in R3

Conclusion Arm microprocessor is simple, low power consuming, and efficient Hardwired logic and the barrel shifter make the ARM very fast also