Process Control - Atari ST series Technical Reference Manual

add.l bsslen(a5),dO
1
#stk+bp,dO
add.
*** Calculate the address of your stack
*** and move it to the stack pointer (a7)
.1
dO,dl
m o v e
a5,dl
add.l
1 2
and. #- ,dl
move.l dl,a7
*** Use the GEMDOS M s h r i n k Q
* * *
actually used for the program and stack, and release the
*** rest back to the free memory pool.
move.l dO,-(sp)
*
move.l a5,-(sp)
*
clr.w ~(sp)
move #$4a,-(sp)
*
trap #1
1 2
add.l # ,sp

Process Control

One of the Disk Operating System's major functions is to
load a program (also known as a process) and start it. After
the program finishes, GEMDOS also has the responsibility
for terminating the process, reclaiming its memory, and re­
turning to the GEM Desktop (or whatever other application
happens to be operating as a command shell). GEMDOS in­
cludes four process control functions, one for loading and ex­
ecuting a process, and three for terminating one.
The function used to load a program file and execute it
is called Pexec(). The syntax for this function is
*char file, command, env;
int mode;
long status;
status = Pexec(mode, file, command, env);
The meaning of the function parameters varies according to
the value of mode. There are four different modes of opera­
tion for the Pexec() function. These are:
Table 5-1. Four Modes for Pexec() Function
Mode
Number Function
0 Load and
execute
100
CHAPTER 5
* - i - length of uninitialized storage segment
* + (size of base page + stack/heap)
new stack address =
size of program memory
+ program's base address,
pick off odd bit to make sure that the
stack starts on a word boundary (it must)
set stack pointer to your stack
which is stk bytes above end of BSS
call to reserve the area of memory
* push the size of program memory
* (first M s h r i n k Q parameter) on the stack,
* push the beginning address of the
* program memory area (2nd Mshrink() parameter)
* clear a dummy place-holder word
* finally, push the GEMDOS command number
* for the Mshrink() function
* call GEMDOS
* and clear your arguments off the stack.
File
Pointer to
filename
string
Command
Pointer to
command string
Env
Pointer to
environment
string