The document discusses an example assembly language program that implements a delay of 0.01 seconds using an 8051 microcontroller. It explains that the program uses nested loops and a subroutine to achieve the fixed, loop, and adjustment times needed for the desired delay. Registers R0 and R1 are used to control the loop counts. The program returns after completing all loops and the subroutine to produce the accurate 0.01 second delay.

1. Chapter 1
Compiling the
First Program
A compiler is a program that processes statements written in a particular programming language
(C, Assembler, etc.) and turns them into machine code executed directly by a computer's Central
Processing Unit (CPU). Typically, a programmer writes language statements one line at a time using
an editor. Below, Keil 8051 Development Tools (C51)© is used for defining the Accumulator
Register.

2. Assembler Program and Machine Code………………….…….………. 1
Machine Code in Program Memory………………..…….……………. 1
Assembler’s Output Files.……………………………………….……. 2
Components of Assembly Language line.…………………………...…. 3
Instruction Set.………………………………………………………. 3
Assembler Directives.…………………………………………..……. 3
Bit, Byte, Word, Binary, HEX…….………………………………….... 4
Style Guidelines.……………………..……………………………... 4
Clock.……………………..……………………………..………... 5
Example Program (Delay).……..…………………………..………... 5
Tips.……..…………………………..…………………….……... 7

3. Assembler Instruction
Machine Code
Operation Code
Flash/EE Program Memory
Program Counter
Assembler Instruction:
Accumulator Register = FEH
Assembler Instruction:
No Operation
Statement: End
Control Statement:
Program Start Address (0000H)
Assembler Instruction:
Jump to Absolute Address
(0100H)
Control Statement:
Next Code defined at
100H of Program Memory
Label: Represents the program
memory address of an instruction
BEGIN_PROG: = 100H
Machine Code is the closest to the hardware
Mnemonic Description Byte Oscillator Periods
MOV A, #data Move Immediate Data
to Accumulator
2 12
Machine Code in Program Memory
General format Description
MOV <dest-byte>, <src-byte> The byte variable indicated by the second
operand is copied into the location specified by
the first operand. The source byte is not
affected. No other register or flags are affected
0 1 1 1 0 1 0 0
The programmer writes a set of assembler instructions
(Assembler Program or Assembly Code) in an editor,
the Assembler translates assembly code to machine
code (sequence of 0's and 1's that constitute the object
program). The output file with the machine code and
other symbols is used to program the Microcontroller.
After Reset, the Microcontroller initializes the Program
Counter (PC, holds the memory address of the next
instruction that would be executed) with the First
Address of Program Memory (0000H) in order to fetch
the first Operation Code (OP) that has the information
of what task the microcontroller must execute.
Assembler Program and Machine Code
Assembly language is a low-level programming language for a programmable device specific to a computer
architecture. The instruction set, used for creating an assembler program, combine mnemonics and syntax that is
translated into their numerical equivalents. This representation typically includes an operation code as well as
other control bits and data. The follow assembler instruction uses 2 bytes in memory and delay 12 clock periods
1
2
7
3
4
5
5
1
6
1
1
2
3
4
1
2
3 4
5
6
7
4
#data
35
0x0101
2
0x0100
High Level Language
Assembly Language
Machine Code
Hardware

4. Assembler’s Output Files
Assembler Instruction Address Program Memory Machine Code
LJMP Address
LJMP 0100H
0000H
0001H
0002H
0 0 0 0 0 0 1 0
0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0
Operation Code (02H)
Program Address (15-8): 01H
Program Address (7 - 0): 00H
MOV A, #data
MOV A, #0FEH
0100H
0101H
0 1 1 1 0 1 0 0
1 1 1 1 1 1 1 0
Operation Code (74H)
#data: 0FEH
NOP 0102H 0 0 0 0 0 0 0 0 Operation Code (00H)
The Assembler generates several output files during compilation. By default, each output file shares the same
filename as the source file but has a different file extension. The most important files are:
.HEX: Output file in Intel HEX file format. Every line follows a specific structure explained below
Number of Data bytes in the Record Address Field Data bytes
: 0 3 0 0 0 0 0 0 0 2 0 1 0 0 F A
Colon Record type 00 Checksum
.LST: The Assembler Program is listed along with the generated instruction opcodes or machine code. Listing file
may optionally contain cross reference.
.OBJ: Object file that must be linked with other object files to produce a complete executable.
Assembler Program and Machine Code
Table 1.1.- Assembler Instructions and equivalent Machine Code stored in Program Memory
Listing (LST) File
1
Assembler Program
Address of Program Memory
2 Machine Code
3
4 Symbols Table
5 Register Banks
1 2 3
4
5
2

5. Label: Assigns a symbolic representation to the address in program memory corresponding to line
Opcode: Reserved symbols that correspond to the operation to be performed by the microcontroller
Operands: Entities operated upon by the instruction
Comment: Used to document/understand programs
Components of Assembly Language line
Instruction Set
Assembler Directives
Assembler Program and Machine Code
Each line of assembly language source code has this general form:
{Label} {Opcode} {Operands} {;Comments}
1
2
3
4
11
2 2
33 4
The Instruction Set of a Microprocessor executes at least the following general operations: Arithmetic and Logical, Data
Transfer, Boolean Variable Manipulation and Program Branching, some examples are shown in the following table
Arithmetic Operations Data Transfer Logical Operation Program Branching Boolean Variable
ADDC A,source MOV dest,#data ORL direct,A JNE @Ri,#data,rel SETB C
SUBB A,#data XCHD A,@Ri CPL A DJNZ Rn,rel CLR bit
DIV AB MOV DPTR, #data16 ANL A,#data LCALL addr16 JNB bit,rel
The operands involved in the instructions are described below
Rn Register addressing using R0-R7
direct 8 bit internal address (00H-FFH)
@Ri Indirect addressing using R0 or R1
source Any of [Rn,direct,@Ri]
dest Any of [Rn,direct,@Ri]
#data 8bit constant included in instruction
#data16 16bit constant included in instruction
bit 8bit direct address of bit
rel Signed 8bit offset
addr16 16bit address
Directives are commands that are part of the assembler syntax but are not related to microcontroller instruction set.
Assembler Directives are used as instruction to the assembler and are no translated to machine code during the
assembly process. Some Instructions to Assembler: Address Control, Conditional Assembly, Memory Initialization and
Reservation, Procedure Declaration, Program Linkage, Symbol Definition, etc. The following table explains the actions
of certain directives
3

6. Bit, Byte, Word, Binary, HEX…
Style Guidelines
EQU Define symbol DW Store word values in program memory
DATA Define internal memory symbol ORG Set segment location counter
IDATA Define indirect addressing symbol END End of assembly source file
XDATA Define external memory symbol CSEG Select program memory space
BIT Define internal bit memory symbol XSEG Select external data memory space
CODE Define program memory symbol DSEG Select internal data memory space
DS Reserve bytes of data memory ISEG Select indirectly addressed internal
DBIT Reserve bits of bit memory data memory space
DB Store byte values in program memory BSEG Select bit addressable memory space
0 1 0 0 1 1 0 1
1 0 0 1 1 1 1 0
Assembler Program and Machine Code
You can use the following style guidelines to improve the readability and understandability of your programs:
✓ Provide a Program Header:
o Purpose of program
o Register usage
o Memory usage
o Interrupts
o Additional Hardware
o Input and Output parameters
o Example
o Author
✓ Each major component or module of program should begin with a comment containing its purpose
✓ Each line must fit on the page
✓ Don’t explain obvious actions of an assembly language instruction: “complement bit”
✓ Start labels, opcode, operands, and comments in same column for each line
✓ Use meaningful symbolic names:
o Mixed upper and lower case for readability: Hex16bits_to_Ascii5bytes, Wait_Keypress…
✓ Subroutine comments:
o Name
o Statement of what it does
o List of its parameters and their roles
o List of its arguments and return values
✓ Do not invent the wheel, use libraries: 32-Bit Math Routines for the 8051, INTEL
bit
byte
word
4

7. Clock
Delay of MOV A,#data. ADuC832 = 12 * System Clock. ADuC842 = System Clock
Example Program
In this example, a delay program of 0.01 sec will be implemented based in the formula:
td = tf + tl + ta
where:
td: Desired time
tf: Fixed time
tl: Loop time
ta: Adjustment time
using the reference program that is shows below
Assembler Program and Machine Code
The Microconverters from Analog Devices (based on 8051 core) have incorporated a PLL that converts 32.768 kHz
watch crystal onto a multiple (512) of this to provide a stable 16,777,216 Hz clock for the system (ADuC842). One
machine cycle is equivalent to one clock cycle (5.9605e-08 sec).
The ADuC832 has a microcontroller core that is 8051 instructions set compatible. One machine cycle is equivalent in
this case to 12 clock cycles (12 * 5.9605e-08 sec = 7.1526e-07).
Mnemonic Description Bytes Clock Periods Machine cycles Single-cycle core
MOV A, #data Move Immediate Data
to Accumulator
2 12
(ADuC 832)
1 1
(ADuC 842)
The first step to develop a program is define the microcontroller core and output files, as it’s shown below
5

8. Fixed Time
Adjust Time
Loop Time
Output in GPIO pin P0.7
// ----- Delay equation: td = (((3*NN + 2) + 3)*MM + 6 + 4 (LCALL))*tclock
DELAY_LOOP:
MOV R1, #255 ; MOV R1,#MM (2 Cycles)
LOOP_1:
MOV R0, #217 ; MOV R0,#NN (2 Cycles)
LOOP_0:
DJNZ R0, LOOP_0 ; 3*NN Cycles
DJNZ R1, LOOP_1 ; 3*MM Cycles
; Adjust time
;#####################################################
MOV R1,#160 ; (2 Cycles)
LOOP_2:
DJNZ R1, LOOP_2 ; 3*160 = 480 Cycles
;#####################################################
RET ; 4 Cycles
1
1
2
2
3
3
1
Assembler Program and Machine Code
2
3
% System frequency
fclk = 16777216; % 16.78MHz
% Desired time (10 msec)
tdes = 0.01;
% Clock cycle
tcycle = 1/fclk;
% Based on (Delay 0.01 sec) ==> ((3*NN + 2) + 3)*MM + 6 + 4
% Defining ==> MM = 255
NN = floor(((((10e-3/tcycle) - 10)/255)-5)/3)
% Real Delay
treal = (((3*NN + 2) + 3)*255 + 10) * tcycle;
% Adjust time
tadjust = tdes - treal;
% Cycles to adjust
Cycles_adjust = floor(tadjust / tcycle)
A program to compute the coefficients of loop time and adjust time is shows below
Being the result
1
NN = 217
Cycles_adjust = 482
6

9. Assembler Program Machine Code
Tips
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Complement bit (Exclusive OR with 1)
01010000
XOR 01000101
00010101
Set bit (OR with 1)
01010000
OR 01000101
01010101
Reset bit (AND with 0)
01010000
AND 01000101
01000101
Divide by two (Rotate to the right) 00001000 (08H)
00000100 (04H)
Multiply by two (Rotate to the left) 01001000 (48H)
10010000 (90H)
Assembler Program and Machine Code
7