ARM r1p3, R4F manual Registers, Register set

Programmer’s Model

2.6Registers

The processor has a total of 37 program registers:

•31 general-purpose 32-bit registers

•six 32-bit status registers.

These registers are not all accessible at the same time. The processor state and operating mode determine the registers that are available to the programmer.

2.6.1The register set

In the processor the same register set is used in both the ARM and Thumb states. Sixteen general registers and one or two status registers are accessible at any time. In Privileged modes, alternative mode-specific banked registers become available. Figure 2-3 on page 2-9shows the registers that are available in each mode.

The register set contains 16 directly-accessible registers, R0-R15. Another register, the Current Program Status Register (CPSR), contains condition code flags, status bits, and current mode bits. Registers R0-R12 are general-purpose registers that hold either data or address values.

Registers R13, R14, R15, and the CPSR have these special functions:

Stack pointer Software normally uses register R13 as a Stack Pointer (SP). The SRS and RFE instructions use Register R13.

Link Register Register R14 is used as the subroutine Link Register (LR).

Register R14 receives the return address when a Branch with Link (BL or BLX) instruction is executed.

You can use R14 as a general-purpose register at all other times. The corresponding banked registers R14_svc, R14_irq, R14_fiq, R14_abt, and R14_und similarly hold the return values when interrupts and exceptions are taken, or when BL or BLX instructions are executed within interrupt or exception routines.

Program Counter Register R15 holds the PC:

•in ARM state this is word-aligned

•in Thumb state this is either word or halfword-aligned.

Note

There are special cases for reading R15:

•reading the address of the current instruction plus, either:

—4 in Thumb state

—8 in ARM state.

•reading 0x00000000 (zero).

There are special cases for writing R15:

•causing a branch to the address that was written to R15

•ignoring the value that was written to R15

•writing bits [31:28] of the value that was written to R15 to the condition flags in the CPSR, and ignoring bits [27:20] (used for the MRC instruction only).

You must not assume any of these special cases unless it is explicitly stated in the instruction description. Instead, you must treat instructions with register fields equal to R15 as Unpredictable.

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