assembly working with registers and variables

Question

I have these variables:

A DB 56h
B DW -123
C DB '$'

Then I have 4 operations:

add C,al
add ax,A
add B,al
add C,ax

Only the first one is the correct one. Can anyone explain me why those 3 operations are wrong? (I know they are wrong because of different sizes but I don't know those sizes)

Yes, it does. But if I have to know which operation is correct without using any software. — dontHaveName, Nov 27 '15 at 20:18
*" I don't know the sizes"*. You do: `DB` is 1 byte, `DW` is 2 bytes, `al` is 1 byte, `ax` is 2 bytes. — Weather Vane, Nov 27 '15 at 20:20
so what's wrong e.g. with add ax,A? ax = 2 bytes, DB - 1 byte; 56h = 2 bytes? — dontHaveName, Nov 27 '15 at 20:24
The assembler told you it's wrong, and you'll have to find another assembler, or processor, if you don't like it. `A` is only one byte, as I mentioned. Normally you have to do arithmetic on same-size operands. — Weather Vane, Nov 27 '15 at 20:36
In particular `add C,ax` is trying to fit "a quart into a pint pot". — Weather Vane, Nov 27 '15 at 20:40

score 0 · Accepted Answer · answered Mar 16 '16 at 15:19

Here the possible ways of creating variables in assembly

 `DB       Define Byte       allocates 1 byte 
  DW      Define Word       allocates 2 bytes
  DD      Define Doubleword allocates 4 bytes
  DQ      Define Quadword   allocates 8 bytes
  DT      Define Ten Bytes  allocates 10 bytes`

Registers are fixed tiny storage spaces inside the processors with which the processor can directly work on
And these are the available registers in x86 based assembly.

  `ax => [ah + al] 
  bx => [bh + bl]
  cx => [ch + cl]
  dx => [dh + dl]`

Of which ax,bx,cx and dx are 16 bit registers that can be referenced as such, or the individual pair of 8 bits can be used separately, and those can be referenced using ah,al,bh,..etc where the h in ah,bh etc. stands for "higher" byte of the corresponding 16 bit registers(ax,bx,cx,dx) and the l in al,bl etc. stands for the lower byte
To do arithmetic operations or transfers between two variables ,they should be of the same size

because the size of DB depends on the string passed in. ie. a byte per character, in the above example the variable C is of 1 byte, which is same as that of al - which is the lower byte of ax register.
To see more info about the allocation of variables in assembly, try this link

assembly working with registers and variables

1 Answers1