Write a program to add two 8- bit numbers OE4H and OD9H. Show how the flag register is affected by the above i

Solved Examples · Example 6

Q. Write a program to add two 8- bit numbers OE4H and OD9H. Show how the flag register is affected by the above instructions.

Karnataka PUCTextbookNumericImportance★★★★★est

27% · 31/115 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — 8051 Arithmetic Operations

All 8051 arithmetic is carried out through the accumulator A, and memory-to-memory arithmetic is never allowed. Four operations are studied, each writing worked ALP programs in the board exam.

Addition uses ADD A,source, computing $A = A + source$ ; the source may be a register, a constant or a memory byte. A carry out of bit 7 sets the carry flag CY, and a carry from bit 3 to bit 4 sets the auxiliary carry AC. For example, $1FH + 5EH = 7DH$ .

Subtraction is done only as subtract with borrow, SUBB A,source, giving $A = A - source - CY$ . For a plain subtraction the carry is cleared first with CLR C. Internally the chip forms the 2's complement of the subtrahend (1's complement then add 1), adds it to A and inverts the carry; afterwards $CY = 0$ means the result is positive and $CY = 1$ means it is negative. …

Start your 14-day free trial to unlock the full solution →

[!TLDR]

Adding 0E4H and 0D9H gives 1BDH; the accumulator keeps the low byte BDH while the ninth bit sets CY = 1, and there is no half-carry so AC = 0.

Concept - addition and the flags. ADD A, source leaves the 8-bit sum in A. If the true sum is larger than FFH, the carry out of bit D7 is stored in the carry flag CY; a carry from bit D3 into bit D4 sets the auxiliary-carry flag AC (used for BCD).

Program:

MOV A, #0E4H   ; load 0E4H into A
ADD A, #0D9H   ; add 0D9H to A

Working:

  0E4H     1110 0100
+ 0D9H   + 1101 1001
-----   ------------
 1BDH   1 1011 1101
``` …

Start your 14-day free trial to unlock the full solution →

←Previous questionExample 5 — Write a program to add two 8-bit number, and store it in R5. The numbe…→Next questionExample 7 — Subtract 23H from 3FH when CY = 0, write the program and solve.

Q.Write a program to add two 8- bit numbers OE4H and OD9H. Show how the flag register is affected by the above instructions.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days