Z80: How to add 16 and 8 bit registers?

Question

How do you add a 16 and a 8 bit register with carry (for example, HL, and A)?

Answer 1

You can't do it directly. You need to copy A into a 16-bit register pair and then do the add:

LD  B, 0
LD  C, A
ADC HL, BC

Answer 2

I would like to point out that the checked response (by Carl Norum) is correct, but not the best answer. The following shows the speed of the two strategies with clock cycles. Using the right solution saves time, and won't destroy a second 16 bit register pair.

  4   ld c,a            4   add a,l
  7   ld b,0            4   ld l,a
  11  add hl,bc         4   adc a,h
                        4   sub l
                        4   ld h,a

However, the solution on the right does take an extra byte of code.

Answer 3

From http://nemesis.lonestar.org/computers/tandy/software/apps/m4/qd/opcodes.html

Add Byte with Carry-In Instructions
8080 Mnemonic Z80 Mnemonic  Machine Code Operation
ADC  M        ADC A,(HL)    8E           A <- A + (HL) + Carry