Watkins-Johnson Company WJ-8718A/MFP Instruction Supplement page 76

WJ-8718A/MFP
3.2.2 READ-ONLY/WRITE-ONLY DEVICES
CIRCUIT DESCRIPTION
Flip-flop and buffer integrated circuits on the MFP-A1, A3, and A4 boards serve as
addressable read-only and write-only devices. The devices allow the microproc.essor to monitor
the status of the receiver's front panel controls and update receiver parameters. If the r·eceiver
is equipped with an optional remote interface, the microprocessor uses read-only and write-only
devices to incorporate remote commands and to transmit monitored receiver parameter data.
3.2.3 MICROPROCESSOR THEORY
The microprocessor, located on MFP-A4, unifies the system by controlling the
functions of memory, read-only, and write-only devices. The processor must be able to access,
decode, and execute the binary-coded instructions in the program. During the execution of the
program, the microprocessor must be able to reference memory, if necessary, and must be able
to recognize and respond to interrupt signals. The microprocessor meets its obligations by
utilizing internal circuits as illustrated in Figure 3-6, the Intel 8085A Microproc·essor Block
Diagram. A brief description of the operation of key functional areas within the micro-
processor follows.
3.2.3.1
Registers
Registers are temporary storage units within the microprocessor. Some registers,
such as the program counter, flag register, stack pointer, and instruction register have
dedicated uses. The accumulator and all other general purpose registers are used for storage of
intermediate data during the execution of instructions. The accumulator is also used during
arithmetic operations.
3.2.3.1.1
Program Counter and Stack Pointer
The instructions that make up the program are stored in EPROM {Erasable
Programmable Read Only Memory) on the MFP-A4 board. The microprocessor references the
contents of the memory in order to determine what action is appropriate. This means that the
processor must know which location contains the next instruction. The processor maintains a
register which contains the address of the next program instruction. This register is the
program counter. Because the microprocessor updates the program counter each time it
fetches an instruction, the program counter is always pointing to the next instruction.
The program is stored in EPROM in numerically adjacent addresses (beginning at
location zero). The lower order addresses contain the first instructions to be executed and the
higher order addresses contain the later instructions. During the course of the instructions, the
program may call, or transfer control to, a subroutine. A subroutine is a program within a
program; often, it is a general purpose set of instructions which must be executed repeatedly
during the program. Memory space is conserved by storing the instructions in an area apart
from the main program and directing the processor to this area as necessary. The subroutine
instructions are also stored in numerically adjacent addresses in EPROM. When a subroutine is
called, the processor is instructed to jump to the lowest subroutine address. After the
subroutine is finished, the processor must resume execution of the mdin program; therefore, the
processor must remember the contents of the program counter at the time the call occurs.
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