CODEWITHSUNDEEP
assemblymotorola68000
Harold B
Harold B

Reputation: 23

Hi, need a little help understanding Motorola in assembler

If I have:

0000 3304    alpha:    dc.w    5,16,4,-2

and my instruction is: sub.w alpha+2,D0

Is my alpha at 0000 33004 or is it at 00 05 00 01 00 04 FE?

Upvotes: 0

Views: 196

Answers (1)

d.moncada
d.moncada

Reputation: 17402

dc.w a = define constant word, where a is the value to define

sb.w a,b = subtract word, where a is the source and b is the destination

Instruction

0000 3304    alpha:    dc.w    5,16,4,-2

is equivalent to:

0000 3304    alpha:    dc.w   05
             alpha:    dc.w   16
             alpha:    dc.w   04
             alpha:    dc.w  -02

To break up a bit more, the first instruction is saying starting at address 0000 3304, define a value of 5 then define a value of 16, then a value of 4 and so on. Since each of these values are instructed as a word, whenever a new value is defined, you must increment the address for that value by hex 2 for two bytes/the length for a word.

After the constants have all been defined, the following values will live at the following addresses

0000 3304 ->  0x05
0000 3306 ->  0x10       
0000 3308 ->  0x04
0000 330A ->  0xFE

What instruction sub.w alpha+2,D0 is saying, is to starting at alpha (address 0000 3304) move 2 bytes and subtract that value at that address from the value in data register D0.

so,

alpha+2 = address 0000 33006

which has a value of 0x10.

Whatever value is in data register D0, subtract 0x10 from it.

Upvotes: 3

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