Radio Shack TRS-80 Model II Reference Manual page 180

Model II
TRSDOS
How to Use the Supervisor Calls
Supervisor Calls
(SVCs)
are Operating System routines available to any user
program. The routines alter certain System functions and conditions; provide
file access; perform I/O to the Keyboard, Video Display, and Printer; and
perform various computations.
All the
SVCs
leave memory above
X'2FFF'
untouched. Only those Z-80
registers used to pass parameters from the
SVC
are altered. All others are
unaffected. However, all the prime registers are used by the System; they are
not restored.
Each
SVC
is assigned a Function Code. These codes run from 0 through 127.
Only the first 96 are defined by the System; codes 96-127 are available for user
definition.
To specify a given Supervisor Call, your program refers to the
SVC's
Function
Code.
Calling Procedure
All
SVCs
are accomplished via the
RST 8
instruction.
1. Load the Function Code for the desiredsvc into the A register. Also load
any other registers which are needed by the svc, as detailed under
"Supervisor Calls."
2. Execute a
RST 8
instruction.
3. Upon return from the
SVC,
the Z flag will be set if the function was
successful. If the Z flag is not set, there was an error. The A register
contains the appropriate error code (except after certain computational
SVCs,
which use the A register to return other information).
Examples
Time-Delay
LD BC,TIMCNT
LD
t\,
6
REiT
8
DELAY OVER-PROGRAM
L.ENG·T"H OF DELAY
FUNCTION CODE
6 -
DELAY-SVC
J"lWIP TO ENC
CON'T" I
NUE~3
HE::
I~E
Output a line to the Video Display
L.D HL, IvlSG
tD B,
1~)
LD
C, it.H)H
LD 1-\,9
RErr 8
,JR NZ, (-:iOTERR
IF NO ERROR THEN PROGRAM
216
POINT TO THE MESSAGE
B=CHARACTER COUNT
C=CTRL CHAR. TO ADD AT END
CODE
9
= DISPL.AY LINE-SVC
,JUJVIP TO SVC
J"UMP IF I/O ERROR
CONT I
1\~IJEf3
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