Texas Instruments 990 User Manual page 89

Applications Programming
4.2.1 Input/Output Calls
Assembly language programs can perform SVCs by setting up SVC
call blocks and executing level 15 XOP instructions using those
call blocks. The call blocks pass the necessary parameters to
the SVC >40 processor.
To execute the communications SVCs, byte 0 of each call block
must contain the opcode >40. The I/O opcode in byte 2 of the
call block spec {fies "the opera tion that is to be performed. For
normal read/write operations, the I/O opcodes that may be used by
applications programs are 2 or 3. For requests for activation
services, the I/O opcode ranges from 4 through 7.
Applications programs should normally use the input and output
calls for the following purposes:
* Executing write calls that transmit data to the
dest ina tion program
*
If a response from the dest ina tion
program is
antic ipated, request ing act ivation se rvices and then
executing a time delay SVC while waiting for the
response
*
When reactivated, executing a read call to retrieve any
input data addressed to it
The following paragraphs define the format of an SVC block for
executing input/output operations.
4.2.1.1 Input/Output Call Block.
The format of the SVC block for an input/output call is shown in
Figure 4-1. The data contained in the call block originates as
follows:
*
Data supplied by the applications program:
SVC
>40
I/O opcode
I/O data buffer pointers
Input and output character counts
Destination
10
(DID), for write operations
Source
10
(SID) when not set to zero by
applications program
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