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15.7 LTL protocol

With most compilers languages a program has to be first compiled (i.e. translated from the source text to machine code) and then linked (i.e. joined up with other program portions and library functions and saved as a program file) before it can be executed. Load-time linking (abbreviated 'LTL') omits the last step until the program is to be executed. Only then does the so-called loader assemble the program, though without saving it as a program file. Thus the linking process takes place in RAM, immediately before the execution of the program.

This text desribes a protocol permitting a shell to communicate with a loader (i.e. a program that implements load-time linking). This protocol was used for the first time by the development environment Chatwin and the Oberon-System STJ-Oberon-2. But the protocol has been constructed in such a way that in principle it can also be used by other shells and loaders.


15.7.1 The OBNL cookie

The communication between shell and loader takes place via a cookie named OBNL. This points to a routine with which the shell can call the loader. Hence the cookie must be created by the loader.

The routine to which the OBNL cookie points is defined as follows: 

int cdecl obnload ( OBNCOMM *com );

i.e. that the routine passes on the stack a pointer to a OBNCOMM structure and that the routine returns a 16-bit value in register d0.

15.7.2 The OBNCOMM structure

The OBNCOMM structure is built up as follows: 

typedef struct _obncomm
{
  int   type;
  void *ptr;
  int   chr;
  void *env;
} OBNCOMM;

The field type contains one message number at a time. Depending on this number, the other entries of the structure are then filled.

'int' is a signed 16-bit integer, 'void *' is a pointer to 'something' i.e. it is not at first specified particularly what type of data the pointer points to.

15.7.3 LTL messages from the shell to the loader

The shell can send the following messages to the loader:

Message Number
CL_INIT 0x6500
CL_COMMAND 0x6501
CL_TIME 0x6502

Messages from the shell to the loader always start with 'CL_'.

Please note: These are not AES messages! The communication takes place via the routine to which the OBNL cookie points.

15.7.3.1 CL_INIT

With the message CL_INIT the shell inform the loader that it supports the LTL protocol. At the same time it passes to the loader the address of a function via which the loader can trigger actions in the shell.

The function in the shell has the same parameters as the function to which the OBNL cookie points: 

int cdecl obnshell ( OBNCOMM *com );

Assignment of the OBNCOMM structure:

type CL_INIT
 
ptr Address of the function in the shell
 
chr Undefined
 
env Undefined
 

15.7.3.2 CL_COMMAND

The shell is to execute a command that requires a load-time linking. After this it passes on the complete command to the loader.


Assignment of the OBNCOMM structure:

type CL_COMMAND
 
ptr Pointer to a NULL-terminated string that contains the complete command that was input (incl. parameters)
 
chr Undefined
 
env Pointer to the environment
 

15.7.3.3 CL_TIME

If there has been no user input for some time and the shell also has nothing to do otherwise, it can inform the loader about this so that it is free to perform tasks in the background if desired.


Assignment of the OBNCOMM structure:

type CL_TIME
 
ptr Undefined
 
chr Undefined
 
env Undefined
 

15.7.4 LTL messages from the loader to the shell

The loader can send the following messages to the shell:

Message Number
LC_WRCHAR 0x6503
LC_WRSTR 0x6504
LC_OUTBUF 0x6505
LC_CLOSEWIN 0x6506
LC_OPENWIN 0x6507

Messages from the loader to the shell always start with 'LC_'.

Please note: These are not AES messages! The communication takes place via the routine that the shell has informed the loader about at CL_INIT.

15.7.4.1 LC_WRCHAR

The loader sends to the shell a character that is to be output on the console.


Assignment of the OBNCOMM structure:

type LC_WRCHAR
 
ptr Undefined
 
chr The character to be output
 
env Undefined
 

15.7.4.2 LC_WRSTR

The loader sends to the shell a NULL-terminated string that is to be output in the console.


Assignment of the OBNCOMM structure:

type LC_WRSTR
 
ptr Pointer to the string to be output
 
chr Undefined
 
env Undefined
 

15.7.4.3 LC_OUTBUF

The loader directs the shell to output to the console immediately any characters that may still be left in the buffer.


Assignment of the OBNCOMM structure:

type LC_OUTBUF
 
ptr Undefined
 
chr Undefined
 
env Undefined
 

15.7.4.4 LC_CLOSEWIN

The loader directs the shell to close all windows that may be open at the time. This call is required at the launch of a GEM module under Single-TOS.


Assignment of the OBNCOMM structure:

type LC_CLOSEWIN
 
ptr Undefined
 
chr Undefined
 
env Undefined
 

15.7.4.5 LC_OPENWIN

The loader directs the shell to reopen all windows closed previously with LC_CLOSEWIN.


Assignment of the OBNCOMM structure:

type LC_OPENWIN
 
ptr Undefined
 
chr Undefined
 
env Undefined
 

15.7.5 Example: Chatwin and STJ-Oberon-2

As an example one should show here briefly how Chatwin (shell) and STJ-Oberon-2 (loader) implement the LTL protocol:

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