2. Load register from memory

2.1. Document conventions

Notation: Rt, Rn denote ARM registers R0-R7 except when stated otherwise. immN represents an immediate value having a width of N bits hence imm5 is constrained to the range 0-31. [Rn + immN] is the contents of the memory address obtained by adding Rn and the offset immN. Offsets are measured in bytes. These instructions affect the condition flags.

2.2. Register Load

  • ldr(Rt, [Rn, imm7]) Rt = [Rn + imm7] Load a 32 bit word
  • ldrb(Rt, [Rn, imm5]) Rt = [Rn + imm5] Load a byte
  • ldrh(Rt, [Rn, imm6]) Rt = [Rn + imm6] Load a 16 bit half word

When a byte or half word is loaded, it is zero-extended to 32 bits.

The specified immediate offsets are measured in bytes. Hence in the case of ldr the 7 bit value enables 32 bit word aligned values to be accessed with a maximum offset of 31 words. In the case of ldrh the 6 bit value enables 16 bit half-word aligned values to be accessed with a maximum offset of 31 half-words.