1

trap()

– Interrupt or Trap(or Exception)– Assembler first– HW interrupt address– 0511– Assembler calls kernel C function

trap()– Parameters in stack– 2673 (dev sp … pc ps)

제 28 강 : trap()

2

trap vector

3

savu retu call trap()

4

5

6

sysent[ ]

countcall

countcall

countcall

countcall

7

sysent[ ]

countcall

countcall

countcall

countcall

8

sysent[ ]

countcall

countcall

countcall

countcall

exit() 1

fork() 2

read() 3

9

2754 [fetch data from user space]fuiword() asm function, (fuword() for d-

space) fetch a word from user spaceThis is instruction (PC) that caused trap

mask it to access sysent[]

callp is pointer to sysent structure

2762 [fetch system call arguments from user space]

2762 for (callpcount) fuword( ) into u.u_arg[ ]

Get system call argument into u -> u.u_arg[]

(“count” is # of sys call arg)2770 u -> u.u_dirp = u -> u.u_arg[0] : value of 1st

argument

2771 trap1(address of system call function)

sys call #chmodk

/* next is system call */

/* return from sys call */

Sys call parameters PC + 2

PC

PC – 2

user a.out

10

input argument is pointer to function

2847 calls this function2848

On normal return, u.u_intflg == 0;If this step is not taken, u.u_intflg

== 1;(see 2845)2772 tests u_intflg & sets u_error

2824 return from trap (system call)

Back to user mode