Page 93 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Unit 6: The 8085 Microprocessor Architecture

Bit D and D of the Accumulator are used for serial I/O and do not affect the interrupts. Bit D = Notes
7 6 6
1 enables the serial I/O and bit D is used to transmit output bits.
7
Figure 6.7: Bit Pattern for the SIM Instruction

RIM is a I-byte instruction that can be used for three functions:
1. To read interrupt masks
2. To identify pending interrupts

3. To receive serial data.
Bit D is used to receive serial data. Bits D , D and D identify the pending interrupts as shown in
7 4 5 6
Figure 6.7. The RIM instruction loads the Accumulator with the following information.
6.2.6 Addressing Modes of 8085

Addressing is the technique to transfer data from memory or I/O ports to or from CPU for
execution of an instruction. There are five type of addressing used in 8085 microprocessor system.
They are:
(a) Immediate Addressing: This is the technique of addressing by which the data to be operated
upon by the CPU is given to processor without using memory locations during transfer. In
this mode 8-bit data is given to processor and uses 2-byte instructions, the first byte is opcode
and second byte is 8-bit data. The 16-bit data can also be loaded by using 3-byte instruction.
The first byte is the opcode while 2°d and 3rd bytes are the address of memory locations.
For example MVI B, 02H loads register B with value 02H and LXI H, 2050H loads register H
with 20H and register L with 50H.

(b) Direct Addressing: It is the mode of addressing in which the address of memory location
where data is residing is given in the instruction. These instructions contain three bytes.
Where first byte is opcode and two bytes are for address of data the lower byte of address is
in byte 2 and higher order byte is in byte 3 of instruction. For example LDA 2050H loads the
Accumulator with data contents available at 2050H, STA 2051H transfers 8-bit data from
Accumulator to 2051H address of memory.
(c) Register Addressing: In this mode of addressing the operands resides in the general purpose
registers the operands are moved within the registers by using opcodes. For example: MOVA,
B moves the contents of the register B into Accumulator and instruction is in the coded
form. This means that the instruction is single byte instruction and opcode itself is having
register address. Another example may be ADD B which adds contents of register B to the
contents of the Accumulator and result is stored in the Accumulator.

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