Register Banks and Stack Memory Allocation in 8051 Microcontroller - LEKULE BLOG


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Wednesday, 24 August 2016

Register Banks and Stack Memory Allocation in 8051 Microcontroller

The collection of general purpose registers (R0-R7) is called as register banks, which accept one byte of data. The bank register is a part of the RAM memory in the embedded microcontrollers, and it is used to store the program instructions. Each microcontroller consists of various memory banks, and each bank register consists of a unique address for recognizing the storage location.

Register Banks in 8051

Register Banks in 8051
Register Banks in 8051

The 8051 microcontroller consists of four register banks, such as Bank0, Bank1, Bank2, Bank3 which are selected by the PSW (Program Status Word) register. These register banks are present in the internal RAM memory of the 8051 microcontroller, and are used to process the data when the microcontroller is programmed.

Switching of Register Banks

By default, the 8051 microcontroller is powered up with register bank 0; and, by using the Program Status Word (PSW), we can switch to other banks. The two bits of PSW are used for switching between the register banks. These two bits are accessed by the bit-addressable instructions SETB and CLR.
Based on the possible combinations of RS1 and RS0 of PSW, the register bank is changed accordingly, i.e., if RS1 and RS0 are 0, then the Bank 0 is selected. Similarly, Bank1, 2&3 are selected as per the values of RS1 and RS0.

Stack Memory Allocation in 8051 Microcontroller

The stack is an area of random access memory (RAM) allocated to hold temporarily all the parameters of the variables. The stack is also responsible for reminding the order in which a function is called so that it can be returned correctly. Whenever the function is called, the parameters and local variables associated with it are added to the stack (PUSH). When the function returns, the parameters and the variables are removed (“POP”) from the stack. This is why a program’s stack size changes continuously while the program is running.

The register used to access the stack is called stack pointer register. The stack pointer is a small register used to point at the stack. When we push something into the stack memory, the stack pointer increases.
Stack Memory Allocation in 8051 Microcontroller
Stack Memory Allocation in 8051 Microcontroller


When an 8051 microcontroller power up, the stack pointer contained value is 07, by default, as shown in the above figure. If we perform ‘PUSH’ operation, then the stack pointer address will be increased and shifted to another register. To avoid this problem, before starting the program, we have to assign a different address location to the stack pointer.

PUSH operation

The ‘PUSH’ is used for taking the values from any register and storing in the starting address of the stack pointer, i.e., 00h by using ‘PUSH’ operation. And, for the next ‘PUSH’, it increments +1, and stores the value in the next address of the stack pointer, i.e., 01h.
PUSH operation of Stack
PUSH operation of Stack

PUSH operation means (First in First out)
Example: WAP in assembly language for PUSH operation
MOV 08h, #21h
MOV 09h, #56h
PUSH 00h
PUSH 01h

POP Operation

It is used for placing the values from the stack pointer’s maximum address to any other register’s address. If we use this ‘POP’ again, then it decrements by 1, and the value stored in any register is given as ‘POP’.
POP Operation in Stack
POP Operation in Stack

POP operation means ‘Last in First out’.
MOV 00H, #12H
MOV 01H, #32H

Registers of 8051 Microcontroller

If we perform any operation whether addition or subtraction, then these operations are unable to be performed directly in the memory, and therefore, are performed by using the registers. There are different types of registers in 8051 microcontroller.

These registers are classified into two types based on their operations:
• General Purpose Registers
• Special Function Registers

General Purpose Registers
As we discussed earlier in this article that there are four different bank registers with each bank having 8 addressable 8-bit registers, and only one bank register can be accessed at a time. But, by changing the bank register’s number in the flag register, we can access other bank registers, which have been discussed earlier on this paper along with interrupt concept in 8051.
Special Function Registers
The special function registers including the Accumulator, Register B, Data pointer, PCON, PSW, etc., are designed for a predetermined purpose during manufacturing with the address 80H to FFH, and this area cannot be used for the data or program storage purpose. These registers can be implemented by bit address and byte address registers.

Types of Special Function Registers

The 8051 consists of four input/output related special function registers in which there are totally 32 I/O lines. The special function registers control the values read from the I/O lines and the special function registers that control the operation of 8051. The auxiliary special function registers are not directly connected to the 8051 – but, in fact, without these registers – the 8051 cannot operate properly. The register set of 8051 is explained below.

Register set of 8051 Microcontroller

Setting a fixed constant value in the register is called a register set. The values are set in the registers using instruction set. The 8051 follows CISC instructions with ‘Harvard’ architecture. The CISC stands for complex instruction set computing. Different types of instructions in the 8051 microcontroller include:

  1. Arithmetic Instructions
  2. Conditional Instructions
  3. Call and Jump Instructions
  4. Loop Instructions
  5. Logical Instructions
  6. Bullion Instructions
1. Arithmetic instructions
The arithmetic instructions perform several basic operations such as:
  • Addition
  • Subtraction
  • Multiplication
  • Division
Arithmetic instructions in 8051 Microcontroller
Arithmetic instructions in 8051 Microcontroller


a. Addition:
Org 0000h
MOV R0, #03H // move the value 3 is register R0//
MOV A, #05H // move the value 5 in accumulator A//
Add A, 00H // accumulator value ‘5’ with 0 and stored on accumulator//

b. Subtraction:
Org 0000h
MOV R0, #03H // move the value 3 is register R0//
MOV A, #05H // move the value 5 in accumulator A//
SUBB A, 03H // A=5-3 final value is 2 stored in the Accumulator A //

C. Multiplication:
Org 0000h
MOV R0, #03H // move the value 3 is register R0//
MOV A, #05H // move the value 5 in accumulator A//
MUL A, 03H // A=5*3 final value is 15, stored in the Accumulator A //

D. Division:
Org 0000h
MOV R0, #03H // move the value 3 is register R0//
MOV A, #15H // move the value 5 in accumulator A//
DIV A, 03H // A=15/3 final value is 5 stored in the Accumulator A //

2. Conditional Instructions

The CPU can execute the instructions based on the condition by checking the single-bit status or byte status is called conditional instructions such as:

To check the single-bit status in bit-addressable register

JB- jump if below

JNB- jump if not above

To check the carry bit status

JC- jump if carry flag

JNC-jump if no carry

To check the accumulator status either 0 or 1

JZ- jump if zero flag

JNZ- jump if not zero

This is all about the register set in 8051 microcontroller and their stack memory allocation. We hope this article might have given you some essential insights about the topic along with some very interesting programs accompanying each topic. You can also write to us for any help in coding the microcontroller and also about the latest projects on microcontroller.
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