A flag that nobody reads changes nothing. Extend the program so that it also reacts to the borrow.

Keep storing the difference at 2002H, and additionally store 01H at 2003H if the subtraction borrowed, or 00H if it did not.

The one thing: The carry flag is the shortest-lived thing in the machine. Branch on it immediately, or turn it into a byte, before anything else overwrites it.

What is being checked

• 2002H still holds the difference
• 2003H holds 01H after a borrow and 00H otherwise
• 2000H and 2001H still hold the original numbers
• The program reaches HLT

Flags are the most perishable thing in the machine

The carry flag is one bit, and almost every arithmetic or logical instruction rewrites it. SUB, ADD, ANA, XRA, RLC — all of them. So the moment after a subtraction is the only moment you can trust the borrow, and if you need it later than that you must turn it into something more durable, which is exactly what this step is doing.

Some instructions are deliberately harmless to the flags, and they are worth memorising because they are what lets you do bookkeeping between a test and its branch:

• MOV, MVI, LDA, STA, LXI — no flags touched at all
• INX and DCX — no flags touched, unlike their 8-bit cousins
• INR and DCR — set the zero, sign and parity flags, but leave carry alone

XRA A is the usual way to clear the accumulator, and it is the one to avoid here: it clears the carry flag along with A.