Motorola EXORdisk II User Manual page 349

CHAPTER 27
27. OTHER SYSTEM FUNCTIONS
In the following description of the system functions
these 'symbols will be used:
Symbol Meaning
A A accumulator
B B
accumulato~
X Index register
CC Condition code
~egister
Z Zero flag of condition code register (bit
2)
c
Carry flag
of
condition code register
(bit 0)
XH
Most significant byte of X
XL Least significant byte of X
B,A
The
register pair B and A treated
as a
sixteen bit register
For MDOS09, the registers
YI
U and DP are unchanged by
the system function calls.
It is assumed that the reader is familiar with
what
system functions are, how they are invoked} what precautions
must be taken
~hen
testing programs using system functionsl
and how errors are handled by system functions
(se~
section
24.8) .
The remainder of this chapter
is
devoted to the
description of all system functions not described thus far.
The description is divided into the following sections:
register functions, double-byte arithmetic functions,
character string functions, diskette file functions, and
miscellaneous functions:
27.1 Register Functions
The register functions are used by some
~f
the other
system functions as an extension of the
M6800
instruction
set. Many operations that involve the transfer and exhange
of information between the register pair
IfB,A"
and the
X
register are made feasible by the fact that the SWI
instruction (used ror accessing system runction handler)
automatically saves all registers on the stack. Since the
sixteen bit
registe~s
are pushed on the stack least
Page
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