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.
PUSH B instruction in 8085 μp causes :
The contents of registers B and C to be copied in the stack.
Two facts settle this: PUSH always moves a register pair, and it copies rather than moves.
The 8085 stack is 16 bits wide in operation, so the four permitted operands — PUSH B, PUSH D, PUSH H and PUSH PSW — each transfer two bytes. Naming "B" is shorthand for the BC pair, and the C register goes along with it. That eliminates option 1.
The registers are not disturbed. Their contents are copied into memory; nothing writes back to them, so B and C hold exactly what they held before. That eliminates options 2 and 4, leaving option 3.
What actually happens, cycle by cycle. The stack grows downward in memory, and the high byte goes first:
| Step | Action |
|---|---|
| 1 | SP is decremented |
| 2 | Register B (high byte) is stored at the new SP |
| 3 | SP is decremented again |
| 4 | Register C (low byte) is stored at the new SP |
So after the instruction the stack pointer has fallen by 2:
\(SP_{new}=SP_{old}-2\)
The matching POP B reverses the sequence — C is retrieved first, then B, and SP rises by 2. PUSH B takes 12 T-states, POP B only 10, the extra time being the two decrements.
Why the stack is used. It is the standard way to preserve registers across a subroutine or an interrupt service routine, so that the called code may use them freely:
PUSH B PUSH D PUSH H PUSH PSW … POP PSW POP H POP D POP B
The order must be reversed on the way out, because the stack is last-in-first-out; getting it wrong swaps register contents and produces bugs that are very hard to trace. Note also that the stack shares ordinary read/write memory, so LXI SP must set the pointer to the top of available RAM before any PUSH is executed, and a deeply nested program can otherwise let the stack grow down into its own data.
Hence, PUSH B causes the contents of registers B and C to be copied in the stack.
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
