1. ARM Microprocessor
“MIPS for the Masses”

2. 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

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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

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