Atari ST series - Page 108

CHAPTER 5
add.l
add. 1
bsslen(a5),dO
#stk+bp,dO
* -i- length of uninitialized storage segment
* + (size of base page + stack/heap)
*** Calculate the address of your stack
*** and move it to the stack pointer (a7)
move.1
add.l
and. 1
dO,dl
a5,dl
#-2,dl
move.l dl,a7
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
*** Use the GEMDOS MshrinkQ call to reserve the area of memory
* * * actually used for the program and stack, and release the
*** rest back to the free memory pool.
move.l dO,-(sp) * push the size of program memory
* * (first MshrinkQ parameter) on the stack,
move.l a5,-(sp) * push the beginning address of the
* * program memory area (2nd Mshrink() parameter)
clr.w ~(sp) * clear a dummy place-holder word
move #$4a,-(sp) * finally, push the GEMDOS command number
* * for the Mshrink() function
trap #1 * call GEMDOS
add.l #1 2 ,sp * and clear your arguments off the stack.
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 File Command Env
0 Load and Pointer to Pointer to Pointer to
execute filename command string environment
string string
100