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msp430carryflag

Carry bit in msp430

I am writing simulator to microcontroller msp430. I cant understand when i should set carry bit. For example in add instruction: 1+0x7FFF setting carry bit or 1+0xFFFF?

Solution

  • For the ADD instruction, the carry bit is set on unsigned overflow.

    You can deduce that from the examples in TI documents. In particular, the second example in the documentation of ADD instruction (page 3-22) says that carry occurs on ADD.B if the result is greater than 0xff (and for ADD and ADDA the limits are 0xffff and 0xfffff respectively - 8, 16 and 20 bits):

    ADD.B           #10,R5     ; Add 10 to Lowbyte of R5
JC TONI                    ; Carry occurred, if (R5)  ≥  246 [0Ah+0F6h]
......                     ; No carry

    The fact that there is a NEGATIVE bit in the msp430 status register in addition to the carry bit confirms this.

    There are at least a couple of existing open-source MSP430 emulators, namely mspsim and Avrora. I suggest to use them as reference implementations.