Add Two 16-bit Numbers
Add numbers that do not fit in a register, first a byte at a time with the carry chain, then in one instruction with a register pair.
Two 16-bit numbers are in memory: the first at 2000H and 2001H, the second
at 2002H and 2003H. Add them and store the 16-bit sum at 2004H and
2005H.
Each number occupies two bytes, low byte first. So the number 1234H is
stored as 34H then 12H. That ordering is not an accident of this exercise —
it is what LXI, LHLD, SHLD and every 16-bit instruction on the 8085
expect, so following the convention means the hardware does the work for you,
and fighting it means doing everything by hand.
The interesting part is that a 16-bit addition is not two independent 8-bit additions. Adding the low halves can produce a carry that belongs to the high half, and passing that carry along correctly is the entire idea. Get it wrong and the program is right for about 99% of inputs, which is the worst kind of wrong.
You will do it twice: once byte by byte, so you can see the carry travel, and once with the 16-bit instructions that do the same thing in a single step.
Steps
- 1 Add It a Byte at a Time
- 2 Add It in One Instruction