The ADD and SUB Instructions

The ADD Instruction

This instruction is fairly simple - it adds two things together. But there are a few limitations and some interesting effects. We'll start with the general form: add [register],[value_to_add].This adds [value_to_add] to [register] and stores the result in [register]. However, [register] must be either a or hl. If [register] is hl, [value_to_add] must be bc, de or hl. So the following are allowed:

Sample_ADD_Instructions:
  ld a,5
  ld b,6
  add a,b
  add a,$12

  ld c,3
  add a,c
  add a,a

  ld hl,$fc00
  ld de,12
  add hl,de

But the following aren't:

Illegal_ADD_Instructions:
  ld b,7
  ld c,9
  add b,c             ; First argument must be a

  ld a,5
  ld hl,12
  add a,hl            ; Can't use a single register with a pair

  ld de,$1234
  ld bc,$0123
  add bc,de           ; First argument must be hl

But what happens in the following example?

Strange_ADD_Code:
  ld a,250
  ld b,100
  add a,b

250 + 100 = 350, but the maximum value for the a register is 255. Using an emulator (that's a complicated tool that isn't really necessary) we can see that the a register contains the number 94 after the program has been run. The number 94 is in fact 350 mod 256 (divide and take the remainder). That way there can never be a number larger than 255 in any 8 bit register. But how do we know if there has been an 'overflow'? The answer is the f (flags) register. If there was indeed an overflow then the carry flag would have been set. This will be explained later.

The SUB Instruction

This instruction is just as simple - it is used for subtraction. The general form is sub a,[value_to_sub], and it makes a become a - [value_to_sub]. Only the a register is allowed as the first argument.