Commodore Plus 4 - Page 239

Machine Language on the
Commodore Plus/4
A computer consists of a central processing unit (CPU), some memory, and
input/output (I/O) circuitry that communicates with the outside world. When
the CPU is a microprocessor, the computer is a microcomputer. The CPU of the
Commodore Plus/ 4 is a 7501 microprocessor. A computer program consists of a
list of instructions to the CPU that command it to perform operations on the
memory and the I/O units. Programming done at this level is called machine-
language programming. Higher-level languages such as BASIC are written in
machine language. An assembler may be used to allow a programmer to use
mnemonics for instructions and labels for memory locations. The term assembly
language refers to machine-language programs that are created with the use of an
assembler.
The 7501 microprocessor understands exactly the same instructions as the
6502 microprocessor. Because this instruction set originated with the 6502, it is
referred to as 6502 machine language.
This chapter should be sufficient for 6502 programmers to transfer their skills
to the Commodore Plus/4. It should also allow programmers of other micropro
cessors to begin programming in 6502. Programmers new to machine language of
any kind may want to refer to additional material on machine-language pro
gramming. Many such books are available.
Introduction to Using Machine Language
Programming in machine language (or machine code) puts your program in
complete control of what the computer is doing. To get this control, some of the
simplicity of programming in BASIC or another high-level language is sacrificed.
In general, programs in machine language are faster and more memory efficient
than programs in BASIC. When these qualities are required, the sacrifice is
worthwhile.
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