F$SSvc modified by Lothan

Lothan — Fri, 27 Jun 2014 23:18:39 -0000

Set SVC
Adds or replaces a system call

OS9 F$SSvc 103F 32

Entry Conditions

Y address of the system call initialization table

Exit Conditions

None

Error Output

CC carry set on error
B error code, if any

Additional Information

Set SVC adds or replaces a system call, which you have written, to NitrOS-9’s user and system mode system call tables.
Register Y passes the address of a table, which contains the function codes and offsets, to the corresponding system call handler routines. This table has the following format:

Relative
Address
Use

$00
Function Code

$02
Offset From Byte 3
To Function Handler

$03
Function Code

$05
Offset From Byte 6
To Function Handler

More Entries

$80
End-of-table mark

If the most significant bit of the function code is set, NitrOS-9 updates the system table. If the most significant bit of the function code is not set, NitrOS-9 updates the system and user tables.
The function request codes are in the range $29-$34. I/O calls are in the range $80-$90.
To use a privileged system call, you must be executing a program that resides in the system map and that executes in the system state.
The system call handler routine must process the system call and return from the subroutine with an RTS instruction.
The handler routing might alter all CPU registers (except Register SP).
Register U passes the address of the register stack to the system call hander as shown in the following diagram:

Relative
Address
Name

U -->
CC

$00
R$CC

A
$01
R$A (R$D)

B
$02
R$B

DP
$03
R$DP

X
$04
R$X

Y
$06
R$Y

U
$08
R$U

PC
$0A
R$PC

krn

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