ARM7 Programmer's Model or Register Model


Registers's Model


  • In total 17(Visible)+20(Banked Rrgisters)=37 
  • The active registers available in the user mode are shown below.
  • This is protected mode which is normally used while executing applications.
  • 16 Data registers & one status register 
  • r0 to  r13 are orthogonal general purpose register.
  • Orthogonal means, any instruction that you can apply to ro can equally be applied to any of the other register.
    • Eg. ADD ro, r1, r2
    • ADD r5, r6, r7
  • R13 (stack pointer) and stores the top of the stack in the current processor mode.
  • R14(LR) Link Register where the core puts the return address on executing a subroutine.
  • R15(PC) Program counter stores the address of next instruction to be executed.
  • In ARM state all ARM instruction  are 32-bits wide.
  • In Thumb state all instructions are 16-bit wide.
  • In ARM state Instruction have to be four byte aligned in the memory. Which implies that the bottom two bits of the PC are always zero(Memory location 1000H,1004,1008H).

CPSR: Current Processor Status Register

About CPSR
  • ARM core uses CPSR to moniter & control internal operations.
  • The unused part reserved for future expansion.
  • CPSR fields is divided in to four fields, each 8-bits wide: flags, status, extension, and control.
  • In current designs status & extension fields are reserved for for future purpose.
  • In some ARM processor cores have extra bits allocated J bit (available only on Jazelle enabled processing which execute 8-bit instructions).
CPSR Diagram


Flag bitSets when

N- Negative         

In case of signed no. operations If result 

MSB=1  ;Indicates the result of operation is NEGATIVE

Z- Zero    The result of operation is zero
C- CarryThe result causes an unsigned carry(carry out of MSB)
V-OverflowThe result causes a signed overflow
Q- SaturationThe result causes an overflow or saturation
I- Interrupt request DisableIf set interrupt request channel is disabled
F- Fast interrupt request Disable If set fast interrupt request channel is disabled
J- Jazelle instruction setIf set processor will execute Jazelle instructions 
T-Thumb instruction set  If set processor will execute Thumb Instruction set

SPSR: Save Program Status Register

Suppose Processor is in USER mode of operation and if IRQ request arrives then processor has to switch itself to IRQ mode of operation but at the same after serving IRQ mode processor should return to USER mode and should resume its working.
So current processor status is copied into SPSR from CPSR in order to resume back.

Difference ARM & Thumb Instruction Set

ARM processor core can support 3 instruction sets. These are
1. ARM Instruction set
2. Thumb Instruction set
3. Jazelle Instruction Set
By default it executes ARM Instruction set

ParametersARM Instruction SetThumb Instruction Set
How Processor Executes?By default Processor Executes
T=0, from CPSR Register
T=1, from CPSR register
Instruction Size / Width32-bit16-bit
No. of CPU instructions5830
Conditional Execution of InstructionsAlmost all instructions get conditionally executedOnly branch Instructions are get conditionally executed
Data Processing InstructionsAccess to Barrel shifter and ALUSeparate barrel shifter and ALU instructions
Program Status Register AccessRead & Write access in privilleged modeNo direct access
Register uses15 General Purpose Register + PC8 General Purpose+ 7 High Purpose + PC


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Last modified: Wednesday, 4 March 2020, 4:56 PM