This article is sort of a prelude to looking at the 8086's hardware. (I was looking at the Intel documentation and realized it was confusing and slightly wrong,

This article is sort of a prelude to looking at the 8086's hardware. (I was looking at the Intel documentation and realized it was confusing and slightly wrong, so needed to figure out what is really happening.) The 8086 implements the instruction in microcode, but like all the arithmetic operations, the interesting stuff happens in the ALU. In particular, the ALU hardware has a DAA instruction that figures out what correction value is needed and adjusts the flags appropriately. Another microcode instruction performs the addition. So the microcode is just four instructions long.