Q.Briefly explain signed and unsigned subprogram

The 8051 supports both unsigned arithmetic (numbers 00H–FFH, no sign) and signed arithmetic (where the most significant bit represents the sign), the difference being how the result and the flags are interpreted. …

[!TLDR]

Unsigned arithmetic treats all 8 bits as magnitude (0–255) and uses the CY flag, while signed arithmetic treats bit D7 as a sign, stores negatives in 2's complement, and uses the OV flag; the 8051 handles both, and signed subtraction with SUBB reads the carry to tell positive from negative.

The 8051 is an 8-bit machine that supports unsigned arithmetic operations directly, but by using the OV (overflow) flag it can carry out both signed and unsigned addition and subtraction.

Unsigned numbers:

• All 8 bits of the byte represent the magnitude, so the range is 0 to 255 (00H to FFH).
• There is no sign; the numbers are always treated as positive.
• The 8051 supports unsigned addition, subtraction, multiplication (MUL AB) and division (DIV AB) directly through its instructions.
• The carry flag (CY) tells the programmer whether the result was too large for 8 bits (a carry out of D7 in addition, or a borrow in subtraction).

Signed numbers:

• The most significant bit D7 is used as the sign bit: 0 means a positive number and 1 means a negative number; the remaining 7 bits give the magnitude.
• Negative numbers are stored in 2's-complement form (take the 1's complement using CPL, then increment with INC to get the 2's complement).
• The overflow flag (OV), not the carry flag, indicates whether a signed result is valid.

Signed and unsigned subtraction: the 8051 has only the SUBB (subtract with borrow) instruction, so to make a plain subtraction the carry is first cleared (CLR C). SUBB performs the subtraction using the 2's-complement method — the CPU takes the 2's complement of the source, adds it to the accumulator, and inverts the carry. Reading the carry then tells the sign of the result:

• If CY = 0 after SUBB, the result is positive. …