460 Lab Assignment #5
460 LAB Assignment #5
DUE: to be posted
1. OBJECTIVES:
fork and exec in MTX
2. REQUIREMENTS:
Implement the following system calls for the MTX system:
===========================================================================
(8). pid = fork() : fork a child process with an identical Umode image;
parent returns child's pid, child returns 0.
(9). r = exec(file) : change image to the specified (executable) file
As usual, return -1 if the syscall fails.
===========================================================================
3. HELPS and SUGGESTIONS:
1. HOWTO fork():
(1). Do the same as kfork() to create a child proc in such a way that when the
child starts to runs (in Kmode), it RESUMES TO goUmode().
(2). Determine the child's segment as ushort child_segment.
Set child's PROC.uss = child_segment, and
PROC.usp = running->usp (See below for WHY?)
But DO NOT load any Umode image (file) to child's segment because it
will have the SAME Umode iamge of its parent (the running proc).
(3). Use get_word()/put_word() to implement a
copy_image(child_segment)
function, which copies the Umode image of running to child_segment.
*********************** NOTE ******************************************
In general, each process should have a Umode image size recorded in its
PROC structure. ufork() should allocate a child Umode image area of exactly
the same size, and copy_image() should only copy the exact amount form
the parent to the child. So far, we have assumed that each proc's Umode
image is a full segment (64 KB), so you may copy the entire 64KB segment.
Later, with memory management, this CAN be changed to suit the Umode
image size of the process that's doing ufork().
***********************************************************************
copy_image() implies that the Umode images of parent and child are
IDENTICAL. So, their saved usp must also be the same value (each is an
offset relative to its own segment).
(4). While copy_image() is easy, the important part is how to fix the
child's ustack contents so that it will
. return to Umode in its OWN SEGMENT, and
. as if it had executed
pid = fork();
but with a return pid = 0
(Recall that the parent returns with the chid's pid)
That's the fun part!!!! Think about how to do these first.
=============================================================================
2. HOWTO exec("filename");
(1). In general:
Locate the file "filename", verify it's eXecutable, read file header
to determine the TOTAL memory needed, allocate a memory area for the
NEW Umode image, then load the EXECUTABLE part of the file into memory.
----------------------------------------------------------------------
For this assignment: all executable files are generated with a 32-byte
header (because the loader in mtxlib expects them to have headers), but
you may ignore the size requirement since each process has a fixed Umode
image size of 64 KB.
----------------------------------------------------------------------
(2). After loading the new Umode image, fix the ustack contents to make the
process execute from virtual address 0 when it returns to Umode. Refer to
the diagram again:
(LOW) uSP | by INT 80 | HIGH
---------------------------------------------------------------------
|uDS|uES| di| si| bp| dx| cx| bx| ax|uPC|uCS|flag| XXXXXX
---------------------------------------------------------------------
-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 | 0
(a). re-establish ustack to the very high end of the segment.
(b). "pretend" it had done INT 80 from (virtual address) 0:
(c). fill in uCS, uDS, uES in ustack
(d). execute from VA=0 ==> uPC=0, uFLAG=0x0200,
all other registers = 0;
(e). fix proc.uSP to point at the current ustack TOP (-24)
Finally, return from exec() ==> goUmode().
Sample Solution: samples/lab5/lab5.gz