This document provides an introduction to the 8051 microcontroller architecture, including its internal registers, instruction set, and memory map. It discusses the programmer and hardware designer views of the 8051. The register set includes accumulators, registers, data pointers, program counters, stack pointers, and status flags. An overview of 8051 assembly programming is provided, including basic syntax, directives, and the design flow from writing assembly code to downloading the executable onto a board. An example assembly program is presented to demonstrate filling registers and calculating their sum.

1. Pari vallal Kannan
Center for Integrated Circuits and Systems
University of Texas at Dallas
8051 Microcontroller –
Architecture, Intro to Assembly
Programming
EE4380 Fall 2002
Class 2

2. 29-Aug-02 2
Class –2: Objective
l 8051 internal architecture
l Register Set
l Instruction Set
l Memory Map
l Intro to Stack, SFRs
l Assembly language programming

3. 29-Aug-02 3
8051 Architecture
l Programmer’s View
– Register Set
– Instruction Set
– Memory map
l Hardware Designer’s View
– Pinout
– Timing characteristics
– Current / Voltage requirements

4. 29-Aug-02 4
Programmer’s View – Register Set
l Registers
– A, B, R0 to R7 : 8 bit registers
– DPTR : [DPH:DPL] 16 bit register
– PC : Program Counter (Instruction Ptr) 16bits
– 4 sets of register bank R0-R7
– Stack pointer SP
– PSW : Program Status Word (a.k.a Flags)
– SFR : Special Function Registers
l Control the on-board peripherals

5. 29-Aug-02 5
Assembly – Absolute Basics
l Intel Assembly format
Operation destination source ; comment
l Values are to be preceded by a # sign
– #55, #32 etc
l Hex values are to be followed by H
– #55H, #32H
l If the first figure in a hex quantity is a letter (A-F) then a
0 must precede it
– #0FFH, #0C1H, #0D2H
l No operation : NOP !

6. 29-Aug-02 6
Register Set – Accumulator A, ACC
l Commonly used for mov and arithmetic
l Implicitly used in opcodes or referred to as
ACC or by its SFR address 0E0H
l Example of Implicit reference
– Instruction : mov A, R0 (copy contents of R0 to A)
– Opcode : E8
– The Accumulator is implicitly coded in the opcode
l Explicit reference to Accumulator
– Instruction : push ACC
– Opcode: C0 E0

7. 29-Aug-02 7
Register Set – B Register
l Commonly used as a temporary register, much
like a 9th R register
l Used by two opcodes
– mul AB, div AB
l B register holds the second operand and will
hold part of the result
– Upper 8bits of the multiplication result
– Remainder in case of division

8. 29-Aug-02 8
Register Set – R0 to R7
l Set of 8 registers R0, R1, … R7, each 8 bit
wide
l Widely used as temporary registers
l Available in 4 banks (effectively 4x8 registers)
l Bank is chosen by setting RS1:RS0 bits in
PSW
l Default bank (at power up) is the bank0

9. 29-Aug-02 9
Registers - DPTR
l 16 bit register, called Data Pointer
l Used by commands that access external
memory
l Also used for storing 16bit values
mov DPTR, #data16 ; setup DPTR with 16bit ext address
movx A, @DPTR ; copy mem[DPTR] to A
l DPTR is useful for string operations, look up
table (LUT) operations

10. 29-Aug-02 10
Registers - PC
l PC is the program counter
l Referred to as the Instruction Pointer (IP) in other
microprocessors
l PC points to the next program instruction always
l After fetching an instruction (1 or multi byte), PC is
automatically incremented to point to the next
instruction
l Cannot directly manipulate PC (exceptions JMP
statements)
l Cannot directly read contents of PC (tricks available)

11. 29-Aug-02 11
Registers - SP
l SP is the stack pointer
l SP points to the last used location of the stack
– Push operation will first increment SP and then copy data
– Pop operation will first copy data and then decrement SP
l In 8051, stack grows upwards (from low mem to high
mem) and can be in the internal RAM only
l On power-up, SP is at 07H
l Register banks 2,3,4 (08H to 1FH) is the default stack
area
l Stack can be relocated by setting SP to the upper
memory area in 30H to 7FH
– mov SP, #32H

12. 29-Aug-02 12
Registers - PSW
l Program Status Word is a “bit addressable” 8bit
register that has all the flags
l CY - Carry Flag
– Set whenever there is a carry in an arithmetic operation
l AC - Aux. Carry Flag
– Carry from D3 to D4. Used for BCD operation
l P - Parity Flag
– P=1 if A has odd number of 1s
– Even parity
l OV - Overflow Flag
– Set if any arithmetic operation causes an overflow

13. 29-Aug-02 13
Flags - Illustration
l Addition example
38 0011 1000
+ 2F 0010 1111
--------- ---------------
67 0110 0111
--------- ---------------
CY = 0
AC = 1
P = 1

14. 29-Aug-02 14
Registers - SFRs
l Control the operation
of on-board
peripherals
l Special Function
Registers at direct
addresses 80H to FFH
l 8051 Clones may have
additional SFRs

15. 29-Aug-02 15
8051 - Memory Map
l Internal ROM is vendor dependant
l On power-up PC starts at 0000H in
ROM space
MOVC
MOVC, MOVX
MOVX A, @DPTR
MOV A, xxH
InstructionSignalEndStartMemory
Type
????H0000HInternal ROM
PSENFFFFH0000HExternal
ROM
RD,
WR
FFFFH0000HExternal RAM
7FH00HInternal RAM

16. 29-Aug-02 16
8051 – Instruction Set
l Data Transfer
– Move/Copy data from one location to another
– mov, movc, movx, push, pop, xch, xchd
l Logical
– Perform logic operations on data
– anl, orl, xrl, clr, cpl, rl, rlc, rr, rrc, swap
l Arithmetic
– Perform arithmetic operations on data
– add, addc, subb, inc, dec, mul, div
l Program control
– Control the program flow (jumps, subroutine calls)
– jmp, ajmp, ljmp, sjmp, jc, jnc, jb, jnb, jbc, jz, jnz, acall, lcall,
cjne, djnz, ret, reti
l NOP

17. 29-Aug-02 17
8051 Assembly Introduction
l Assembly statement structure
[label:] opcode [operands] [;comment]
l Example
start: mov A, #D0H ;code starts here
l Assembler directives
– ORG xxxxH : origin, start assembling at xxxxH
– EQU : define a constant
l count EQU 25
– DB : define byte, defines data
l DATA1: DB 28
l DATA2: DB “hello world”
– END : end of assembly file

18. 29-Aug-02 18
Assembly Design Flow
l Create the assembly source file test.asm
l Assemble the asm file
– as51 test.asm
– Assembler produces error and code list in test.lst
– If no errors, assembler produces .obj file
l Link the .obj files to produce an .abs file
l Create hex file from the .abs file
l Most assemblers directly produce the .hex file
l Download the .hex file onto the board or burn it into an
eprom.

19. 29-Aug-02 19
Assembly Example #1
l Target 8051 dev system
– Std 8051 device
– 2K on-chip ROM running
a monitor program
– 32K external RAM at
address 0x0000 to
0x7FFF
– This RAM is both code
and data
– First 0x30 locations in
external RAM is
dedicated for the Interrupt
Vector Table (IVT)
l Program to fill up the first 4 registers in the
register bank with some numbers and find their
sum
ORG 0x30 ;skip the IVT area
Start: mov R0, #10
mov R1, #0A5H
mov R2, #1
mov R3, #0x20
clearA: mov A, #0 ;now A = 0
Addup: add A, R0 ;now A = A + R0
add A, R1
add A, R2
add A, R3
mov R4, A ;store sum in R4
mov DPTR, #7FFF
movx @DPTR, A ;store in ext. mem
Done: sjmp done ;loop here forever
END

20. 29-Aug-02 20
Class –2 Review
l What are the different views/models of a uP ?
l What are the registers available in the 8051 ?
l What are the functions of the 8051 registers ?
l What is stack, PC, SFR, PSW/Flags ?
l What is an instruction set ?
l What is a memory map ? Why is it needed ?
l What is an assembly language program ? How
does it look ?