Theory Of Operation; Mc6809E Cpu - Radio Shack TRS-80 Service Manual

Color Computer 2 Service Manual
- - - - - - - - - - - T R S - B O
® - - - - - - - - - - -
4/ Theory Of Operation
4.1 MC6809E CPU (U23)
The heart of any computer system is the Central Processing
Unit, or CPU. In the Color Computer 2, as well as most
modern microcomputers, the CPU is a single Large Scale
Integrated Circuit (LSI). The CPU gathers instructions and
data from memory, interprets and executes the instructions,
and stores the results of the data operations into memory.
Additionally, the CPU stores data to and retrieves data from
various input/output CI/0) devices.
The Motorola MC6809E Microprocessor is perhaps the most
powerful 8-bit microprocessor available today. There are
several ways to determine the "size" of a microprocessor
<whether it is 8-bit, 16-bit, 32-bit, or whatever). One way
involves the number of data interconnecting lines the
processor possesses. Another is the size of the internal
registers and the size of the mathematical and logical
operations supported by the processor. Although the MC6809E
has an 8-bit data bus, internally it contains four 16-bit
registers and two additional 8-bit registers which may be
linked together to form another 16-bit register. The
MC6809E also supports some 16-bit mathematical and logical
operations. Therefore, although it is technically an 8-bit
processor, it has some of the power of the 16-bit machines.
Figure 4-1 is a "programmer's model" of the MC6809E CPU.
Following is a brief description of the registers and their
respective functions. Additional information may be
obtained from the MC6809E data sheet.
The Program Counter CPC) register is a 16-bit register. It
is the job of the program counter to keep track of which
instruction is next. Instructions for the MC6809E processor
are from one to five bytes in length. As each instruction
is fetched, its specific bit pattern allows internal logic
in the CPU to determine the length for that instruction.
'!'he PC is then immediately updated, even before the
remainder of a multi-byte instruction is executed.
The Hardware Stack Pointer (S) is another 16-bit register.
Its job is to "point" to a specific area of memory that is set
aside by the systems programmer for the specific use of the
CPU. From time to time, the CPU finds it necessary to
"remember" more than it has internal capacity to store. In
these cases, it "pushes the data onto the stack". Later, when
- - - - - - - - - - l l a d 1 e l h a e l l - - - - - - - - - -
- 19 -