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Section A:
System interface
A.2. System call conventions
All
system calls have the same convention:
Register C is loaded with a
function code, additional parameters (if any) are loaded into register E
or
intO register pair DE,
respectively.
Results from the
system
are
returned
in register pair HL,
if any.
Moreover,
register A is loaded
with the contents of L,
and register
8
with the contents of
H,
if
not
stated
otherwise.
The call itself is performed
by
a subroutine CALL to
address 0005H.
For example, if a character is to be sent to the screen,
the system call would look like this:
LD
LD
CALL
C,2
E,•*•
5
function code
output an asterisk
SYSTEM CALL
The system maintains a stack area of its own.
It is thus not
necessary
for the user program to supply extra stack for system calls.
EOS System manual
Daeumling & Zimmermann
Page 4
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