A microprocessor is a semiconductor chip fabricated with entire central processing unit (CPU) on it. It is a programmable device that accepts binary data from an input device, processes the data according to the instructions stored in the memory and provides results as output. Basically microprocessor performs two functions - (a) Fetches an instruction from the memory and (b) Performs the operation specified by the instruction. There are special inputs to the microprocessor called interrupts. External devices use these interrupts to get the microprocessor attention. A micro computer can be built by using 8085 microprocessor along with many other chips such as 8155, 8255, 8279, 8253, 8257, 8259, 8251 etc. 8085 microprocessor is 8-bit microprocessor which has 8-bit data registers where as 8086 is a 16-bit microprocessor.
In 8085 microprocessor, the value of the most significant bit of the result following the execution of any Arithmetic or Boolean instruction is stored in :
The sign status flag
The 8085 flag register holds five flags, and each watches a different property of the result.
| Flag | Bit position | Set when |
|---|---|---|
| S — Sign | D7 | The MSB (bit 7) of the result is 1 |
| Z — Zero | D6 | The entire result is 00H |
| AC — Auxiliary carry | D4 | A carry passes from bit 3 to bit 4 |
| P — Parity | D2 | The result has an even number of 1s |
| CY — Carry | D0 | A carry leaves bit 7, or a borrow is needed |
The sign flag is simply a copy of bit 7 of the result, which is exactly what the question describes, so the answer is option 1.
Why bit 7 is called the sign. In two's-complement arithmetic an 8-bit byte represents −128 to +127, and the most significant bit carries the sign: 0 for positive, 1 for negative. Copying that bit into a flag lets the programmer branch on the sign of a result without any further testing, using JM (jump if minus) or JP (jump if plus).
The distinction from the carry flag is the usual trap. The carry flag records a carry out of the byte — information that has left the result — whereas the sign flag records a bit that is still inside it. Example:
MVI A, 70H ADI 20H → result 90H
Here CY = 0, because nothing carried out of bit 7, but S = 1, because the result's bit 7 is 1. Read as signed numbers, 112 + 32 has overflowed past +127 into −112 — which is precisely why signed and unsigned interpretations must never be mixed.
A practical note. The flags are the upper byte of the PSW (program status word), so PUSH PSW saves the accumulator and all five flags together — essential at the start of an interrupt service routine, since almost any instruction there would otherwise destroy the flags the interrupted program was relying on. Note also that logical instructions such as ANA and ORA affect the flags too, and that INR and DCR deliberately leave the carry flag untouched.
Hence, the MSB of the result is stored in the sign status flag.
Similar Questions
A microprocessor is a semiconductor chip fabricated with entire central processing unit (CPU) on it. It is a programmable device that accepts binary data from an input device, processes the data according to the instructions stored in the memory and provides results as output. Basically microprocessor performs two functions - (a) Fetches an instruction from the memory and (b) Performs the operation specified by the instruction. There are special inputs to the microprocessor called interrupts. External devices use these interrupts to get the microprocessor attention. A micro computer can be built by using 8085 microprocessor along with many other chips such as 8155, 8255, 8279, 8253, 8257, 8259, 8251 etc. 8085 microprocessor is 8-bit microprocessor which has 8-bit data registers where as 8086 is a 16-bit microprocessor.
The following 8051 assembly program is given. Answer the questions based on it.
Line No. Mnemonics 1 MOV SP, #4F H 2 SET B PSW.3 3 MOV R0, #25H 4 MOV R1, #0AH 5 MOV R2, #06H 6 PUSH 8 7 PUSH 9 8 PUSH 0AH . . . V V 20 POP 8 $D_7$ $D_6$ $D_5$ $D_4$ $D_3$ $D_2$ $D_1$ $D_0$ In 8255 programmable peripheral interface device, if the port A and port B are to be set in a hand shake mode along with port C, what are the bits in the 8 bit control word to be set as 0 1 0.
The number system for the machine code of the microprocessor 8085 is
Which of the options in a multipurpose instruction used to implement the interrupt of 8085 and serial data input ?
Both 8155 and 8255 programmable peripheral interface ICs have the following common features :
(i) Programmable I/Os
(ii) Either port A or Port B can be set as either input or output ports.
(iii) One 14-bit down counter
(iv) AD0 – AD7 are multiplexed address/datalines.Each instruction in an assembly program has the following fields :
(i) Lable field
(ii) Operand field
(iii) Comment field
(iv) Mnemonic fieldPlease write the proper sequence of fields :
The mnemonic of 8085 processor indicate :
List – I List – II a. RLC i. Rotate Accumulator Right through carry b. RRC ii. Rotate Accumulator Left through carry c. RAR iii. Rotate Accumulator Left d. RAL iv. Rotate Accumulator Right Codes :
Match the following :
List – I (Pin terminals) List – II (Applications) a. SID, SOD i. Wait state b. READY ii. Serial data transfer c. TRAP iii. Address Latch Control d. ALE iv. Interrupt Codes :
Assertion (A) : In serial communication system, when the transmission of data goes in both ways, it is called full-duplex system. Now if two micro processors are connected in full duplex mode, the amount of data transmitted will be double to the amount of data in half-duplex mode connection.
Reason (R) : When the transmission of data goes in one way, it is called half-duplex system and when the data moves in both ways, it is called full duplex system.
Important Questions from Microprocessors
An 8086 address bus is a/an _____ -bit bus
Program counter for any counter:
In the pin out configuration of the 8085 microprocessor, the sequential input data is represented by
Which of the following functions is not performed by a microprocessor?
Match the columns.
Pin
Description
a. D 0-D 7
1. Reset Input
b. RESET
2. Data Lines
c. A 0,A 1
3. Internal Address
