Pointers and the LD instruction

The LD Instruction

LD is short for load, and that is what the LD instruction does - it loads values from registers into memory, from memory into registers and from registers to other registers. It has the general form ld [from],[to]. For example:

Sample_LD_Commands:
  ld a,12              ; load a register with value 10
  ld b,a               ; load b register with value of a register
  ld c,$2c             ; load c register with value $2c
  ld de,$fc00          ; load de with $fc00

  ld hl,($fc00)        ; load hl with value at memory location $Fc00
  ld ($fc01),hl        ; load memory location $fc01 with value of hl

  ld hl,$1234          ; load hl register with value $1234
  ld a,0               ; load a register with value 0
  ld (hl),a            ; load memory location pointed to by hl with value of a

Pointers

You may have noticed that ld a,$12 and ld a,($12) appear to do different things. They do - the first loads the a register with the value $12 and the second loads the a register with the value at memory location $12.

Similarly, you can do ld a,(hl) (this only works for some registers). This loads the a register with the value stored in the memory location pointed to by hl. So, if hl was $d748 then the a register would get the value stored in memory location $d748, but if hl was $fc00 then the a register would get the value stored in memory location $fc00.

Note that the value of the source for the LD instruction remains unchanged, for example:

Start_Of_Code_Sample:
  ld a,5               ; a = 5
  ld b,a               ; b = a, so b = 5. Register a remains unchanged (= 5).

Also, in ld (hl),a and similar the value of hl remains unchanged, but the value at memory location hl does change.