2.4 Cpu registers in the msp430

The MSP430 has 16 CPU registers. Of these 16, the upper 12 are general purpose 16 bit registers (R4-R15). The lower four are:

• R0 Program Counter(PC) – This register controls the next instruction to be executed by the MSP core. In general, this register is incremented automatically during execution. It can be used as a source in operations normally.
• R1 Stack pointer (SP) – The stack pointer is used to keep track of previous execution modes and to return from interrupts. Can be read as a normal register.
• R2 Status Register (SR) – The status register can be written to change the operating mode of the MSP as specified in the User’s Guide. When read it can act as a constant generator. Depending on the instruction code options this register will be read as: a normal register, 0x0000, 0x0004, or 0x0008 depending on the As bits.
• R3 Constant Generator II – This register cannot be written to, and when read produces: 0x0000, 0x0001, 0x0002, or 0xffff depending on the As bits.