| THE 8085 MPU |
The Microprocessor is a programmable logic device,designed with registers, flip flops, and timing elements.The 8085 (8085A) is an 8 bit general purpose microprocessor capable of addressing 64K of memory.It has 40 pins ,requires a +5V single power supply, and can operate with a 3 MHz single phase clock.
The 8085 is a complex IC of sequential circuits. The sequential circuits are designed to do some operation depending on what is the input on their lines. The vital inputs on the lines are what therefore determines what operation will be done by the sequential circuits inside it. The operations can be very complex and therefore this chip is also called a processor. Since we can find a way to put different values of inputs to the input lines of the processor at different times, we can make it execute different operations in a sequence that we desire. Thus, in other words we can make the processor execute a programme to do useful things for us. These inputs then could alternately, be called instructions. The inputs that we talked of so far are actually voltages to represent 1s and 0s. They can only be 1s or 0s, they are called digital values. The number of input lines that the processor provides are some estimation of the largeness of the instruction set that it can support.
The 8085 has a 8 bit input (actually called the input lines of the data bus), meaning that we could have only a maximum of 2^8=256 different combination of 1s and 0s as inputs to the processor. Well! The 8085 supports almost that many different instructions. The programme that we desire to be executed must be loaded into consecutive locations of memory chips. The memory is not part of the 8085 processor. The memory chips are again sequential circuits consisting of flip flops which are capable of storing digital values. Since we would be interested in storing a huge number of such digital values, a large number of these memories are packed together with a scheme of addresses, so that we can address them individually. Generally the memories are arranged in large numbers of 8 bit bunches each.
The addressing is also done by the voltages (1s or 0s) on the address bus. The 8085 has a address bus which is 16 bit wide. Therefore it can put 2^16 different digital values on it, and therefore it can address a maximum of 2^16 different address locations. This is called the addressing space and it is 64 kilobyte for the 8085, because 2^16= 65535. And then we ask the processor to execute those instructions from a particular memory location onwards. It goes on executing those instructions one after another. And that is all it does. The memory location from which it is to pick up the next instruction for execution is maintained in an internal memory location (Register) called Programme Counter (PC). As would be expected, it has to hold a 16 bit memory address. But we will always need to start from somewhere. As soon as the power is turned on, the 8085 does a reset of its programme counter. It is reset to 0000H ,on start up. After which it floats that value on the address bus. The address bus is connected in a parallel fashion to the entire memory. When the voltages on the address bus are indicating at 0000H, only that memory location is activated. And whatever is the content of that memory location is now floated on the data bus. The data bus feeds that value back to the processor 8085. This is also called a opcode fetch cycle. (an instruction fetch cycle). The 8085 executes that instruction and waits for the next instruction. Apart from executing instructions from consecutive memory locations, the processor can make changes in the value of the program counter itself so that it will fetch the next instruction from some other memory location. This is how jumps are executed. Your programme could also write to the memory locations whose address you specify. The processor uses certain internal memory locations called Registers in doing all the operations that we ask it to do. The contents of those memory locations can be directly altered by the instructions that we give.
8085 Assembly Language Programming
An Assembly Language Program is a set of instructions written in the mnemonics of a given microprocessor.These instructions are the commands to the microprocessor to be executed in the given sequence to accomplish a task.
1.Program to add two one byte numbers
3.Simulation Of Throw Of Dice Using Interrupts
Program to add two one byte numbers
Address Mneumonic Hex Code
F000
LXIH F100 21,00,F1
MOV A,M 7E
LXIH F101 21,01,F1
MOV B,M 46
ADD B
88
STA F102 32,02,F1
HLT
76
Address Mneumonic Hex Code
F000
MVI A,18 3E,18
SIM
30
EI
FB
MVI A,00 3E,00
STA FFF8 32,F8,FF
F009
b:MVI B,00 06,00
MVIC,00
0E,00
F00D
r:MOVA,B 78
STA FFF7 32,F7,FF
MOV A,C 79
STA FFF9 32,F9,FF
PUSH B C5
CALL UPDAT CD,D3,06
CALL UPDAD CD,BC,06
POP
B
C1
CALL :d
CD,3C,F0
MOV A,C 79
ADI
01
C6,01
DAA
27
MOV C,A 4F
CPI
60
FE,60
JNZ
:r
C2,0D,F0
MOV A,B 78
ADI
01
C6,01
DAA
27
MOV B,A 47
CPI
60
FE,60
JZ:b
CA,09,F0
MVI C,00 0E,00
JMP:r
C3,0D,F0
F03C
d:STANDARD DELAY PROGRAM
OR ANY OTHER DELAY PROGRAMS.
Simulation Of Throw Of Dice Using Interrupts
Address Mneumonic Hexcode
F000
EI
FB
MVI A,18 3E,18
SIM
30
F004
r:MVI A,01 3E,01
F006
n:INR
A
3C
CPI
06
FE,06
JNZ:n
C2,06,F0
JMP:r
C3,04,F0
FFB1
PUSH PSW F5
STA FFF9 32,F9,FF
CALL UPDAT CD,D3,06
POP PSW F1
EI
FB
RET
C9
8051 Microcontroller, The: Hardware, Software, and Interfacing by James W. Stewart, Kai X. Miao
Computer Organization - Introduction
8086 Microprocessor - Introduction
8086 Assembly Language Programs
All the above Programs were executed on
Intels SDK-85 Microprocessor Kit.
© Copyrights Madhu Sudan Rao G.K