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A Brief Overview of the PRIMER Hardware
MEMORY
The 8085 microprocessor can access 65536
individual memory locations in the range 0 to 65535 in
decimal (0 to FFFF in hex) but only one at a time.
There
are
two
types
of
memory
in
most
microprocessor based systems, memory that can be
read but not written to, which is called Read Only
Memory (ROM), and memory that can be read from
and written to, which is called Read/Alter Memory or
Random Access Memory (RAM). Both RAM and
ROM chips have address pins which are connected to
the microprocessor's address pins. These pins are
connected through what is called an address bus and
through this bus the microprocessor can select a
memory location
for writing or reading of data.
Writing to ROM chips has no effect.
The RAM and ROM chips in the PRIMER trainer have
eight pins which send data to, or receive data from
the microprocessor through a group of eight
connections called the data bus. Since there are 8
pins in the RAM and ROM chips, this allows numbers
from 0 to 255 (0 to FF in hex) to be read from or
written to each memory location.
INPUT/OUTPUT
The 8085 microprocessor can also send data to and receive data from chips other than the RAM or ROM. When the
microprocessor wants to perform input or output it disables the RAM and ROM chips and sends an input/output (I/O)
address to the address bus. The I/O address is only 8 bits but it appears on the lower 8 bits (A0-A7) and the higher 8 bits
(A8-A15) of the address bus simultaneously. Since the address generated is only 8 bits long, only I/O addresses from 0-
255 (0-FF hex) can be selected. Most microprocessor-based systems have circuitry which decode the address from the
address bus and select the appropriate I/O device. Usually these devices are dedicated to either input only or output only.
If an input device has been selected, 8 bits of data is transmitted from the input device to the data bus and into the
microprocessor. If an output device has been selected, the 8 bits of data is sent from the microprocessor to the data bus
and to the output device.
The control bus is a group of connections which provide control over reading or writing of memory or I/O devices. Below is
a block diagram showing the way the CPU (microprocessor) connects to the memory and I/O devices through the address
bus, data bus and control bus.
REGISTERS
The 8085 microprocessor has within it temporary storage devices called registers. Registers work similarly to RAM in that
they store binary values. The 8 bit general purpose registers provided by the 8085 are named A, B, C, D, E, H and L. The
A register is often referred to as "the accumulator".
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