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                    === SvarCOM implementation notes ===

=== SWAPPING =================================================================

While conventional RAM is scarce, a command line interpreter must make effort

to reduce its memory footprint when launching applications. SvarCOM does that

by installing a small executable module in memory, called RMOD (for Resident

MODule). SvarCOM pre-sets RMOD so knows how to execute the external program

and removes itself from memory, letting RMOD do the job. RMOD executes the

application, waits for it to finish and then calls back SvarCOM.

=== NLS STRINGS ==============================================================

SvarCOM can output information in many languages. To do so, it relies on a

precompiled resource file named SVARCOM.LNG. When SvarCOM starts, it looks

for this file in the %NLSPATH% directory and loads from it the part that

contains the %LANG% language. All this is done by nls_langreload().

The SVARCOM.LNG file is compiled by a separate tool: TLUMACZ. It takes

CATS-style language files as input and compiles them into a single SVARCOM.LNG

resource file. It also produces a DEFAULT.LNG with english strings only, this

one is embedded into the SvarCOM executable to display english text in case

SVARCOM.LNG is unavailable.

=== BATCH FILES SUPPORT ======================================================

When SvarCOM executes a command, it checks first if it has a *.BAT extension.

If so, it switches into 'batch-processing' mode:

 - allocates a "batch context" structure and attach it to rmod

 - writes the batch filename into the batch context (rmod-owned) memory, along

   with a counter that holds the offset of the next line to be executed.

 - a batch context has a "parent" pointer that may point to another batch

   context (owned by a different batch instance), it is, in essence, a linked

   list that allows batch files to call one another (typicall through the CALL

   command) allowing SvarCOM to get back to the parent batch once the child

   terminates.

When the rmod batch context pointer non-NULL, SvarCOM does not ask the user for

a command. Instead, it opens the batch file, jumps to the "next line to be

executed" and loads the command from there, incrementing the line counter in

the process.

=== PIPING COMMANDS ==========================================================

Piping a command means redirecting its standard output (stdout) to the

standard input (stdin) of another command. While redirection of file handles

is a concept well supported by the DOS kernels, piping is not, in part due to

the mono-task nature of DOS. SvarCOM provides piping support through following

logic:

1. user-entered (or batch-acquired) command line is analyzed for any kind of

   redirections (incl. pipes) by redir_parsecmd(). If the command appears to

   be piped, then redir_parsecmd() enforces a stdout redirection to a

   temporary file and moves all the pipe chain to an RMOD-owned buffer named

   "awaitingcmd", appending an stdin redirection so the next command's stdin

   is fed from the temporary file. The command is then executed.

2. before further execution, SvarCOM looks into its "awaitingcmd" buffer, and

   if it is non-empty, it runs its content.

3. when loading commands from the awaitingcmd, SvarCOM sets a special

   "delete_stdin_file" flag and passes it to command-executing functions so

   these remember to delete the stdin-redirected file.

=== GLOBAL EXECUTABLE LINKS ==================================================

SvarCOM features special support for "global executable links". This allows to

run selected programs from any directory, without the need to copy these

programs to a directory in %PATH%. Executable links are flat files written in

%DOSDIR%\LINKS\. Each file there contains the directory where the matching

program should be looked for.

===================================================================== EOF ====