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Intel 8085 microprocessor architecture
Memory
Program, data and stack memories occupy the same memory space. The total addressable memory size is 64 KB.
Program memory - program can be located anywhere in memory. Jump, branch and call instructions use 16-bit addresses, i.e. they can be used to jump/branch anywhere within 64 KB. All jump/branch instructions use absolute addressing.
Data memory - the data can be placed anywhere as the 8085 processor always uses 16-bit addresses.
Stack memory is limited only by the size of memory. Stack grows downward.
First 64 bytes in a zero memory page should be reserved for vectors used by RST instructions.
Interrupts
The 8085 microprocessor has 5 interrupts. They are presented below in the order of their priority (from lowest to highest):
INTR is maskable 8080A compatible interrupt. When the interrupt occurs the processor fetches from the bus one instruction, usually one of these instructions:
- One of the 8 RST instructions (RST0 - RST7). The processor saves current program counter into stack and branches to memory location N * 8 (where N is a 3-bit number from 0 to 7 supplied with the RST instruction).
- CALL instruction (3 byte instruction). The processor calls the subroutine, address of which is specified in the second and third bytes of the instruction.
RST5.5 is a maskable interrupt. When this interrupt is received the processor saves the contents of the PC register into stack and branches to 2Ch (hexadecimal) address.
RST6.5 is a maskable interrupt. When this interrupt is received the processor saves the contents of the PC register into stack and branches to 34h (hexadecimal) address.
RST7.5 is a maskable interrupt. When this interrupt is received the processor saves the contents of the PC register into stack and branches to 3Ch (hexadecimal) address.
Trap is a non-maskable interrupt. When this interrupt is received the processor saves the contents of the PC register into stack and branches to 24h (hexadecimal) address.
All maskable interrupts can be enabled or disabled using EI and DI instructions. RST 5.5, RST6.5 and RST7.5 interrupts can be enabled or disabled individually using SIM instruction.
I/O ports
256 Input ports
256 Output ports
Registers
Accumulator or A register is an 8-bit register used for arithmetic, logic, I/O and load/store operations.
Flag is an 8-bit register containing 5 1-bit flags:
- Sign - set if the most significant bit of the result is set.
- Zero - set if the result is zero.
- Auxiliary carry - set if there was a carry out from bit 3 to bit 4 of the result.
- Parity - set if the parity (the number of set bits in the result) is even.
- Carry - set if there was a carry during addition, or borrow during subtraction/comparison.
General registers:
- 8-bit B and 8-bit C registers can be used as one 16-bit BC register pair. When used as a pair the C register contains low-order byte. Some instructions may use BC register as a data pointer.
- 8-bit D and 8-bit E registers can be used as one 16-bit DE register pair. When used as a pair the E register contains low-order byte. Some instructions may use DE register as a data pointer.
- 8-bit H and 8-bit L registers can be used as one 16-bit HL register pair. When used as a pair the L register contains low-order byte. HL register usually contains a data pointer used to reference memory addresses.
Stack pointer is a 16 bit register. This register is always incremented/decremented by 2.
Program counter is a 16-bit register.
Instruction Set
Instruction set of Intel 8085 microprocessor consists of the following instructions:
- Data moving instructions.
- Arithmetic - add, subtract, increment and decrement.
- Logic - AND, OR, XOR and rotate.
- Control transfer - conditional, unconditional, call subroutine, return from subroutine and restarts.
- Input/Output instructions.
- Other - setting/clearing flag bits, enabling/disabling interrupts, stack operations, etc.
Addressing modes
Register - references the data in a register or in a register pair.
Register indirect - instruction specifies register pair containing address, where the data is located.
Direct.
Immediate - 8 or 16-bit data.
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8085 microprocessor architecture
functioning of ALU in 8085
the arithmetic and logical unit will be performing *8 bit addition with /with out carry *8bit substraction with /with out borrow *2 bit BCD(Binary coded decimal) addition *16 bit binary addition*16 bit logical operations-OR,AND,EX-OR,COMPLEMENT and BIT SHIFT OPERATIONS
address/data buffer
the address bus will be having 16 address lines[A15-A0] .In which A7-A0 are called as lower addressing lines and these are multiplexed with data lines[D7-D0] to form multiplexed address /data buffer .The address/data buffer is the bidirectional bus.
address buffer
the remaining higher order address lines form the address buffer ranging from[A15-18].This is having the unidirectional buffer
Instruction register
there are 74 instructions in 8085 .these instructions are classified in to 5 addressing modes they are 1)immediate addressing mode 2)register addressing mode 3)direct addressing mode 4)indirect addressing mode 5)implied addressing mode
serial i/o control
these are control signals used for controlling 8085
these are subdivided into 2 types
1)SID(serial input data):this is used for transferring of data into the memory serially
2)SOD(serial output data):this is used for transferring of data from memory to external devices
Intel 8085 microprocessor
PSW-Five bits are indicate the set of flag and three bits are indicate undefined.The combination of 8bit is programble status word.
Instruction resister or Decoder:-Once the instruction is fetch from thememory,it is reloaded in the instruction resistor for some time, after the decoder decode the instruction performing some event or task.
Resistor Array:-It consists of several types of resistor such as general purpose resistor, stack pointer,& program counfer (PC).There are 6,8 bit general purpose resistor those are B,C, D, E, H, L, all the resistor are called program
components of 8085
the architecture of 8085 consist various components like.
1:Register.
2:ALU.
3:Instruction decoder and machin cycle encoder.
4:Address buffer.
5:Address/data buffer.
6:Inc/Dec latch.
7:Interrupt controll.
8:Serial i/o like SOD,SID.
9:Timing and Controll circuit.
Control and Status Signal:
Two signals RD/WR. Three status signals io/m s1,s0 and special signal called Address enable latch(ALE).
ALE: It is used to separate the multiplexed Address/Data lines into lower order address line(A7A0) and data lineD7D0.A positive going pulse is generated every time the 8085 begins operation.
RD: Active low, Indicates that memory device is ready to be read.
WR: Active low, Indicates that data on the data bus are ready too be written.
IO/M: Differentiate between I/O operation and memory operations.
HIGH :I/O operation is carried on.
LOW: Memory operation is carried on.
s1,s0:Similar to IO/M but are rarely used.
Some important meanings to be kept in mind,Being an Er.
Memory--
Any system needs a facility for storing unprocessed,Partially processed and processed data, this subsystem is known as Memory.
16*4 memory means, it has 4 address lines to have16 addresses.
Lets watch out the architecture of 8085..