Subtract 21H from 30H and write the program and solve.

Exercise Problems · Q5

Q. Subtract 21H from 30H and write the program and solve.

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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. …

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[!TLDR]

Clear the carry, load 30H into A and SUBB 21H; A holds 0FH and CY = 0 marks a positive result.

Concept. The 8051 provides only SUBB A, source (subtract with borrow), so CLR C is used first to force CY = 0 and give an ordinary subtraction. After SUBB, CY = 0 means the result is positive.

Program:

CLR C          ; CY = 0
MOV A, #30H    ; A = 30H
SUBB A, #21H   ; A = A - 21H
``` …

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Q.Subtract 21H from 30H and write the program and solve.

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