Texas Instruments 990 User Manual page 88

Applications Programming
4.2 SUPERVISOR CALLS
Applications programs interface with the communications package
by issuing SVCs that request the communications package to
perform specific functions. SVCs are included in the operating
system to perform I/O, task control, service functions, memory
control, and file management functions. SVCs are explicitly used
in assembly language tasks, whereas higher-level language
statements are processed by the interpreter or compiler and are
translated to the particular call required to perform the
requested operation. If a statement to access a particular SVC
is not available in the higher level language, it is possible to
write an assembly language routine to access the SVC. Examples
of assembly language routines that perform SVCs to the
communications package in FORTRAN or Pascal programs are provided
in this section.
SVCs are implemented using the Model 990 Computer assembly
language extended operation (XOP) instruction. The XOP number is
level 15, and the effective address of the XOP instruction is a
SVC block address. Execution of the XOP instruction passes
control to the operating system. The supervisor call block
referenced in the XOP instruction contains the parameters
necessary for the requested operation to be performed by the SVC
>40 processor. For more detailed information on the XOP
instruction, refer to the Model 990 Computer 990/10 and 990/12
Assembly Language Reference Manual.
Six SVCs can be accessed from applications programs for
communications purposes. The functions they perform and the
methods by which they are accessed are as follows:
Function
Write output
data
Read input
data
Request
activation
services
Assembly Language
Access
SVC >40 with an I/O opcode
of 2
SVC >40 with an I/O opcode
of 3
SVC >40 with I/O opcodes
of 4 through 7
4-2
FORTRAN or Pascal
Access
Call to subroutine
WRIT40
Call to subroutine
REA040
Call to subrou tine
WKUP40
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