Page 11 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Unit 1: Microprocessors and Microcomputers

particular program. The CPU’s 12-b Program Counter addresses up to 16 ROM chips. Simple Notes
applications use only one ROM chip; the desktop calculator used four. The same chip mask also
configured each ROM port bit as an input or output.
Additionally, the ROM chip had an integrated address register, an output data register,
multiplexors, and control and timing logic. The specialized RAM chip had similar resources.

(B) RAM Chip (4002)
Calculators need to hold several 16-digit decimal floating-point numbers. We organized the RAM
accordingly, and ended up with a 20-digit word (80 b):
16 digits for the fraction
2 digits for the exponent
2 digits for signs and control

20 digits × 4 b/digit

Figure 1.4: RAM Chip (4002)

The RAM chip stored four 80-b numbers and additionally the chip had an output port. The use of
three-transistor dynamic memory cells made the RAM chip feasible. A built-in refresh counter
was used to maintain data integrity. Refresh took place during instruction fetch cycles, when the
RAM data was not being accessed. Dynamic RAM memory cells were also used inside the CPU
for the 64-b index register array and 48-b Program Counter/stack array. Intel expertise in dynamic
memory was an enabling factor for the MCS-4!
(C) Input/Output Ports
To conserve chip count and to utilize existing power/clock pins, the 16-pin ROM and RAM chips
also had integrated 4-b ports for direct connection of I/O devices. To activate an output, a program
selected a particular RAM/ROM chip (using an index register) and sent 4-b of accumulator data
from the CPU to the selected output port. In the desk calculator application, the display, keyboard,
and printer were connected to these ports. Keyboard scanning, decoding, and debouncing were
all done under program control of the I/O ports; all printer and display refresh was done in
firmware. A small shift register (4003) was used for output port expansion. External transistors
and diodes were used for amplification and isolation.

Figure 1.5: Input/Output Ports

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