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Homework Help: ARM Cortex-M Machine Code Help

  1. Oct 8, 2013 #1
    1. The problem statement, all variables and given/known data
    Hey guys. I am just learning machine code for Cortex-M. There are a few exercises within the textbook, however, I am not entirely sure if I am doing them correctly. If you could possible look over my work to see if there are any mistakes, I would greatly appreciate it! They are supposed to be relatively simple, however, I am still not entirely sure.

    Each line represents a specific set of machine code. I will provide the code adjacent in brackets.

    0x45-----------(MOV R0, #0x45)
    0x45 0x02------(MOV R1, #0x02)
    0x45 0x02 0x47 (ADD R2, R0, R1)
    0x45 0x02 0x43 (SUB R2, R0, R1)
    0x45 0x02 0x43 (SUBS R2, R0, R1)
    0x45 0x02 0x05 (AND R2, R0, #0x01)
    0x45 0x02 0x05 (ORR R0, R0, #0x01)
    0x0F 0x02 0x05 (MOV R0, #0x0F)
    0x0F 0x0F 0x05 (MOV R1, #0x0F)
    0x0F 0x0F 0x05 (CMP R1, R0)
    0x0F 0x0F 0x00 (SUBS R2, R0, R1)

    The other part had to do with with flags. We are given A and B, two 8-bit numbers, where R = A + B. We have to determine what R would equal, and what the values of the flag registers would be (NZVC).

    A---------- B---------- R---------- NZVC
    10--------100---------110-------- 0000
    100-- ----(-100)-------0---------- 0100
    110-------146---------0---------- 0101
    50---------206-------- 0---------- 0101
  2. jcsd
  3. Oct 8, 2013 #2


    Staff: Mentor

    What's your question? Were you supposed to indicate the values of the three registers for each of the op codes above?
  4. Oct 8, 2013 #3
    Hi Mark44,

    The table was left blank and we were merely given the code indicated in brackets. I then filled out the table accordingly. For the second portion, R = A + B. We were supposed to fill out R and indicate what value the flag registers would be. Sorry I was no more clear.
  5. Oct 8, 2013 #4


    Staff: Mentor

    I'm not familiar with Cortex-M, so caveat emptor.

    Most of your entries in the table look OK, but there are a few that I'm uncertain of.

    For this one -- (SUB R2, R0, R1) -- I'm guessing that the CPU calculates R0 - R1 and stores the result in R2. If so, then what you have for the registers looks fine.

    For this one -- (SUBS R2, R0, R1) -- I don't know what the opcode SUBS means, signed subtract?

    For this one -- 0x45 0x02 0x05 (ORR R0, R0, #0x01) -- I suspect this is different from OR, but I don't know how it's different. In any case, it looks to be ORing what's in R0 with 0x01, which would just result in the same thing, since what's in R0 is odd.

    For the table with flag values, what were you given and what did you fill in?
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