Tswitch
Clients for Tswitch
Clients for the F06 Assignment
This demo shows how
stack changes interact with register save and restore.
Since the stack pointer is saved and restored, a
regrest call can change which stack the
CPU is using, which can change the values
of variables stored on the stack. The output is:
egad(15)
A: arg = 15, r = 0
B: arg = 15
C: arg = 77
A: arg = 15, r = 1
E: arg = 15
egad => 15
D: arg = 77
E: arg = 77
egad => 77
#include <stdio.h>
#include <stdlib.h>
#include <tswitch.h>
unsigned newstack1[512];
regbuf_t buf1, buf2;
int egad(int arg)
{
printf("egad(%d)\n", arg);
int r = regsave(buf1);
printf("A: arg = %d, r = %d\n", arg, r);
if(r == 0) {
printf("B: arg = %d\n", arg);
restack(newstack1, sizeof newstack1, 2, 8);
// ** Now we're on the new stack.
arg = 77;
if(!regsave(buf2)) {
printf("C: arg = %d\n", arg);
regrest(buf1, 1); // ** Returns to old stack.
} else
// Here we're on the new stack, since regsave
// restored the stack pointer.
printf("D: arg = %d\n", arg);
}
// We're on the old stack the first time we pass this, and on the
// new stack the second time.
printf("E: arg = %d\n", arg);
return arg;
}
main()
{
int ret = egad(15);
printf("egad => %d\n", ret);
if(ret < 20) regrest(buf2, 2);
exit(0);
}
Dr. Bennet
Save And Restore