Registers and Processor Modes

The ARM processor has twenty seven registers, some of which have conditions applied, so you only get to use sixteen at any one time...

• Register 0 to register 7 are general purpose registers and can be used for ANY purpose.
  Unlike 80x86 processors which require certain registers to be used for stack access, or the 6502 which places the result of mathematical calculations in the Accumulator, the ARM processor is highly flexible in its register use.

• Register 8 to register 12 are general purpose registers, but they have shadow registers which come into use when you switch to FIQ mode.

• Register 13 is typically the OS stack pointer, but can be used as a general purpose register. This is an operating system issue, not a processor issue, so if you don't use the stack you can corrupt this freely within your own code as long as you restore it afterwards. Each of the processor modes shadow this register.

• Register 14 is dedicated to holding the address of the return point to make writing subroutines easier. When you branch with link (BL) the return address is stored in R14.
  Likewise when the program is first run, the exit address is stored in R14. All instances of R14 must be preserved in other registers (not really efficient) or in a stack. This register is shadowed across all of the processor modes. This register can be used as a general purpose register once the link address has been preserved.

• Register 15 is the program counter. It holds the status of the processor as well as a twenty-six bit number which is the address of the program currently being used.

To make this a little clearer... Another chart:

User Mode  SVC Mode   IRQ Mode   FIQ Mode  APCS

R0 ------- R0 ------- R0 ------- R0        a1
R1 ------- R1 ------- R1 ------- R1        a2
R2 ------- R2 ------- R2 ------- R2        a3
R3 ------- R3 ------- R3 ------- R3        a4
R4 ------- R4 ------- R4 ------- R4        v1
R5 ------- R5 ------- R5 ------- R5        v2
R6 ------- R6 ------- R6 ------- R6        v3
R7 ------- R7 ------- R7 ------- R7        v4
R8 ------- R8 ------- R8         R8_fiq    v5
R9 ------- R9 ------- R9         R9_fiq    v6
R10 ------ R10 ------ R10        R10_fiq   sl
R11 ------ R11 ------ R11        R11_fiq   fp
R12 ------ R12 ------ R12        R12_fiq   ip
R13        R13_svc    R13_irq    R13_fiq   sp
R14        R14_svc    R14_irq    R14_fiq   lr
------------- R15 / PC -------------       pc

The Program Counter is built up as follows:

  Bit  31  30  29  28  27  26  25------------2  1  0

       N   Z   C   V   I   F   Program Counter  S1 S0

By now you may be wondering about these "modes", such as "FIQ" mentioned above.

• User Mode, the usual mode for applications to run in. Your memory access is restricted and you cannot read directly from hardware devices.

• Supervisor Mode (SVC Mode), used mainly by SWIs and the OS. This mode has additional privileges which allow greater control of the computer. For example, you have to go to Supervisor Mode in order to read from a podule. It cannot be done in User Mode.

• Interrupt Mode (IRQ Mode), used to handle peripherals that issues interrupts. This mode is also privileged. Such devices causing IRQs are the keyboard, the VSync (when the screen refresh is occurring), IOC timers, serial, harddisc, floppy etc etc...

• Fast Interrupt Mode (FIQ Mode), used to handle peripherals that issue fast interrupts. This mode is also privileged. Such devices causing FIQs are the floppy disc handling data, the serial port (on 82C71x machines such as the A5000) and Econet.

Refer to psr.html for details of how to change processor modes.