8085 Microprocessor:

The salient features of 8085 μp are :

1. It is a 8 bit microprocessor.
2. It is manufactured with N-MOS technology.
3. It has 16 bit address bus and hence can address up-to
   216 = 65536 bytes (64KB) memory locations through A0-A15.
4. The first 8 lines of address bus and 8 lines of data-bus are multiplexed AD0 – AD7.
5. Data bus is a group of 8 lines D0 – D7.
6. It supports external interrupt request.
7. A 16 bit program counter (PC).
8. A 16 bit stack pointer (SP).
9. Six 8-bit general purpose register arranged in pairs: BC,DE, HL.
10. It requires a signal +5V power supply and operates at 3.2 MHZ single phase clock.
11. It is enclosed with 40 pins DIP ( Dual in line package ).

Memory:

• Program, data and stack memories occupy the same
memory space.

The total addressable memory size is 64KB.

• 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 processor always uses 16-bit addresses
so that data can be placed anywhere.

• 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 processor has 5 interrupts.

They are presented below in the order of their priority (from lowest to highest):
• INTR is mask able 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 mask-able 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 mask-able 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 mask-able 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 mask-able interrupt. When this interrupt is received the processor saves the contents of the PC register into stack and branches to 24H (hexadecimal) address.

• All mask-able 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.

 

Reset Signals

• RESET IN : When this signal goes low, the program
counter (PC) is set to Zero, μp is reset and resets the
interrupt enable and HLDA flip-flops.
• The data and address buses and the control lines are 3-
stated during RESET and because of asynchronous nature
of RESET, the processor internal registers and flags may
be altered by RESET with unpredictable results.
• RESET IN is a Schmitt-triggered input, allowing
connection to an R-C network for power-on RESET delay.
• Upon power-up, RESET IN must remain low for at least
10 ms after minimum Vcc has been reached.

• For proper reset operation after the power – up duration,
RESET IN should be kept low a minimum of three clock
periods.

• The CPU is held in the reset condition as long as RESET
IN is applied.

Typical Power-on RESET RC values R1 =75KΩ, C1 = 1μF.
• RESET OUT: This signal indicates that μp is being reset.
This signal can be used to reset other devices.

The signal is synchronized to the processor clock and lasts an integral
number of clock periods.

Serial communication Signal
• SID – Serial Input Data Line: The data on this line is
loaded into accumulator bit 7 when ever a RIM instruction
is executed.
• SOD – Serial Output Data Line: The SIM instruction
loads the value of bit 7 of the accumulator into SOD latch
if bit 6 (SOE) of the accumulator is 1.
DMA Signals
• HOLD: Indicates that another master is requesting the use
of the address and data buses.

The CPU, upon receiving the hold request, will relinquish the use of the bus as soon
as the completion of the current bus transfer.

• Internal processing can continue. The processor can regain the bus only after the HOLD is removed.
• When the HOLD is acknowledged, the Address, Data RD,
WR and IO/M lines are 3-stated.
• HLDA: Hold Acknowledge : Indicates that the CPU has
received the HOLD request and that it will relinquish the
bus in the next clock cycle.
• HLDA goes low after the Hold request is removed. The
CPU takes the bus one half clock cycle after HLDA goes
low.
• READY : This signal Synchronizes the fast CPU and the
slow memory, peripherals.

• If READY is high during a read or write cycle, it indicates
that the memory or peripheral is ready to send or receive
data.
• If READY is low, the CPU will wait an integral number of
clock cycle for READY to go high before completing the
read or write cycle.
• READY must conform to specified setup and hold times.
Registers
• Accumulator or A register is an 8-bit register used for
arithmetic, logic, I/O and load/store operations.
• Flag Register has five 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/rotation.
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 decremented/ incremented by 2 during push and pop.

• Program counter is a 16-bit register.

Instruction Set

8085 instruction set consists of the following instructions:

1. Data moving instructions.
2. Arithmetic – add, subtract, increment and decrement.
3. Logic – AND, OR, XOR and rotate.
4. Control transfer – conditional, unconditional, call subroutine, return from subroutine and restarts.

5. Input/Output instructions.
6. 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.