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Assembly Language Fundamentals
69
Symbolic Operations
You have been introduced, in small doses, to symbols throughout the chapters preceding this
one. The idea of symbols in an assembly language is the same as
it
is in a higher language such
as BASIC - to make operations simpler and the code more understandable.
Several symbolic tools are provided for you in this assembly language system. You have"
already seen one described in detail in this chapter - labels. There are some pre-defined
symbols the assembly system provides for certain locations in the machine (mostly registers).
There are ways to define your own symbols (and give them a "type"). And, there are ways to
access symbols in other modules.
Symbols can be used as operands in machine instructions and in some pseudo-instructions.
They can be part of expressions in an operand.
Pre-Defined Symbols
The assembler has pre-defined a number of symbols and has reserved them as references to
special locations in memory. Each of the locations has a special meaning and function. The
symbols themselves are "reserved", meaning they cannot be re-defined (by using them as
labels on something else). The symbols are -
Symbol
A
Arl
Ar2
B
Base_page
C
Cb
D
Db
Dmac
Dmama
Dmapa
End _isr _high
End isr low
Isr _flag
Isr_psw
Description
Arithmetic accumulator
} BCD arithmetic accumlators
Arithmetic accumulator
Global temporary area (50 words)
Stack pointer
Address:.extension bit for byte pointer in C
Stack pointer
Address-extension bit for byte pointer in D
DMA count register
DMA memory address register
DMA peripheral address register
Reserved symbols for writing interrupt service routines
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