F$SSvc

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	<-- First entry
$01 $02	Offset From Byte 3 To Function Handler
$03	Function Code	<-- Second entry
$04 $05	Offset From Byte 6 To Function Handler
	More Entries	<-- 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

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