The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

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| TYPE VECTOR SYSDATA NAME MODULE/MODDCB |
| ---- -------- ---------- ---- ________________ |
| | | |
| 10 FCB-FILE PRINTER/06 PR | PRINTER/*PR | |
| |________________| |
| 80 31-bytes of FCB data FCB |
| |
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Figure 3-5: DCB Table After ROUTE
Let's now follow an output request to the *PR device. The DOS device I/O handler will
recognize the ROUTE bit (bit-4 of the TYPE byte) and update the register linkage so that
the FCB will be pointed to instead of the DCB. Noticing that the control block now
indicates a disk file (bit-7 of the TYPE byte), the I/O handler will pass control to the
character I/O file routines.
The action taken by the operating system to reset a DCB that has been routed is to first
close the file, if a filespec was the initial "route-to", then recover the original TYPE
and VECTOR from the SYSDATA field.
3.4.5 Filtering a Routed Device
Let's suppose we have a text file that needs line feeds removed (it may be a CP/M file
that uses CR-LF as the end-of-line protocol). We could write a program to read the file
and write out to another file all characters that are not a line feed. We could also use
a trap filter that is handy. We want to be able to filter the file with this trap filter.
Using the routing identical to that shown in figure 3-5, establish the trap filter and
invoke it with:
SET *LF USING TRAP (CHAR=10)
FILTER *PR USING *LF
Figure 3-6 will now reflect the DCB structure after this series of commands. It is now
easy to LIST the source file with the (P,T=N) option. This will direct a copy of the file
to the *PR device (while suppressing tab expansion). As can be observed from the figure,
the device handler passes *PR I/O requests to the TRAP filter. After performing whatever
filtering is necessary, the @CHNIO request will reference the *LF Device Control Block
(which is pointed to by the MODDCB field). The device handler then notes that the ROUTE
bit is set and continues to control the @PUT request as was done under figure 3-5. A
simple "RESET *PR" upon completion will close the filtered FILE/TXT.
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| TYPE VECTOR SYSDATA NAME MODULE/MODDCB |
| ---- -------- ----------- ---- ________________ |
| | | |
| 47 TRAP PRINTER/06 PR | PRINTER/*PR | |
| |________________| |
| 80 31-bytes of FCB data FCB ________________ |
| | | |
| 10 FCB-FILE LF | TRAP/*LF | |
| |________________| |
| |
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Figure 3-6: Filtering a Route
3.4.6 Linking
Linking is handled by establishing a link Device Control Block storage area for each LINK
command invoked. For example, if you "LINK *DO TO *PR", we can illustrate the DCB area as
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