将汇编语言翻译成英语（EASy68K）

Question

Was given the following code in class and am supposed describe what each line means in comments to the right. Is this correct?

       MOVE.B  #20,D0     //Move 20 into D0
       MOVEA.L #$1000,A0  //Move the contents of address 1000 into A0 
       CLR.B   D1         //Set D1 to 0
Again  CMP.B   (A0)+,D2   //Compare A0 to D2, then increment A0 by 1
       BNE     NEXT       //If A0 and D2 are not equal, go to NEXT, otherwise continue
       ADD.B   #1,D1      //Add 1 to D1
NEXT   SUB.B   #1,D0      //Subtract 1 from D0
       BNE     Again      //Branch to AGAIN if contents of A0 is not equal to D2

Answer 1

No, it's not correct. At the very least, this:

Again  CMP.B   (A0)+,D2   //Compare A0 to D2, then increment A0 by 1

...is not comparing the content of A0 to anything. It's comparing a byte at the address contained in A0 to a byte in D2 (then incrementing A0 to point to the next address).

If I'm not mistaken, in these lines:

NEXT   SUB.B   #1,D0      //Subtract 1 from D0
       BNE     Again      //Branch to AGAIN if contents of A0 is not equal to D2

The zero-flag should be set/cleared based on the result of the immediately preceding sub.b , so it's continuing for 0x20 iterations (because D0 was loaded with 0x20 in the first line).