PPT On 8085 Microprocessor Architecture
Presentation Transcript:1. The 8085 Microprocessor Architecture
2. The 8085 and Its Busses
The 8085 is an 8-bit general purpose microprocessor that can address 64K Byte of memory.
It has 40 pins and uses +5V for power. It can run at a maximum frequency of 3 MHz.
The pins on the chip can be grouped into 4 groups:
Control and Status Signals.
Power supply and frequency.
3. The Address and Data Busses
The address bus has 8 signal lines A8 – A15 which are unidirectional.
The other 8 address bits are multiplexed (time shared) with the 8 data bits.
So, the bits AD0 – AD7 are bi-directional and serve as A0 – A7 and D0 – D7 at the same time.
During the execution of the instruction, these lines carry the address bits during the early part, then during the late parts of the execution, they carry the 8 data bits.
In order to separate the address from the data, we can use a latch to save the value before the function of the bits changes.
4. The Control and Status Signals
There are 4 main control and status signals. These are:
ALE: Address Latch Enable. This signal is a pulse that become 1 when the AD0 – AD7 lines have an address on them. It becomes 0 after that. This signal can be used to enable a latch to save the address bits from the AD lines.
RD: Read. Active low.
WR: Write. Active low.
IO/M: This signal specifies whether the operation is a memory operation (IO/M=0) or an I/O operation (IO/M=1).
S1 and S0 : Status signals to specify the kind of operation being performed .Usually un-used in small systems.
5. Cycles and States
From the above discussion, we can define terms that will become handy later on:
T- State: One subdivision of an operation. A T-state lasts for one clock period.
An instruction’s execution length is usually measured in a number of T-states. (clock cycles).
Machine Cycle: The time required to complete one operation of accessing memory, I/O, or acknowledging an external request.
This cycle may consist of 3 to 6 T-states.
Instruction Cycle: The time required to complete the execution of an instruction.
In the 8085, an instruction cycle may consist of 1 to 6 machine cycles.
6. A closer look at the 8085 Architecture
Previously we discussed the 8085 from a programmer’s perspective.
Now, lets look at some of its features with more detail.
7. The ALU
In addition to the arithmetic & logic circuits, the ALU includes the accumulator, which is part of every arithmetic & logic operation.
Also, the ALU includes a temporary register used for holding data temporarily during the execution of the operation. This temporary register is not accessible by the programmer.
8. The Flags register
There is also the flags register whose bits are affected by the arithmetic & logic operations.
The sign flag is set if bit D7 of the accumulator is set after an arithmetic or logic operation.
Set if the result of the ALU operation is 0. Otherwise is reset. This flag is affected by operations on the accumulator as well as other registers. (DCR B).
This flag is set when a carry is generated from bit D3 and passed to D4 . This flag is used only internally for BCD operations. (Section 10.5 describes BCD addition including the DAA instruction).
After an ALU operation if the result has an even # of 1’s the p-flag is set. Otherwise it is cleared. So, the flag can be used to indicate even parity.
9. Memory interfacing
There needs to be a lot of interaction between the microprocessor and the memory for the exchange of information during program execution.
Memory has its requirements on control signals and their timing.
The microprocessor has its requirements as well.
The interfacing operation is simply the matching of these requirements.
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