System Overview - Commodore VIC-20 Programmer's Reference Manual

SYSTEM OVERVIEW
This chapter provides an overall functional description of the VIC
20 and ties hardware and software operations together to give the
programmer more of an understanding of the way VIC 20
processes his programs within the system.
A simplified functional block diagram of the computer is shown in
Figure 1·1. The major system components include the micropro-
cessor, the program-storage read-only memories (ROMs), the
data-storage random-access memories (RAMs), the versatile
interface devices (VIAs, 6522), the character generator chip
(2332), and the VIC chip which provides video and sound for the
display.
The 6502 microprocessor is the most complex device on the
electronics printed circuit board. This device is primarily responsi-
ble for controlling all computer operations. These operations are
controlled by addressing programs in the read-only memory
(ROM), and then interpreting and executing these sequential
program instructions. The interpretation and execution of instruc-
tions are accomplished during the processor's fetch and execute
cycles. In the fetch cycle, a program instruction is "fetched" into the
processor's instruction register. The program counter (indicates
the location of the instruction in ROM) is counted up, ready for the
next instruction in sequence to be fetched into the instruction
register. In the execute CYCle, the processor executes the
instruction which performs the operation indicated. Addresses
indicating the destination of data being transferred are derived from
the instruction, or calculated using program data and data from the
internal registers.
These controls exercised by the processor are performed by
communicating through the 16-bit address bus, the a-bit bi-direc-
tional data lines, and the write-enable line. The information on the
address bus determines the destination of the data being
transferred, the bi-directional data bus functions as a path for data
transferred into and out of the microprocessor, and the write-enable
line determines the direction of the data being transferred.
Consider the microprocessor's inputs and outputs. We can
divide these into three groups. Each of these groups forms a "bus"
which consists of a set of parallel paths used to transfer binary
information between the devices in the system.
The address bus is used to carry the address generated by the
microprocessor to the address inputs of the memory and
input/output (1/0) devices.
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