minix 3.1's jmp statement problem

ITchimp · **Joined:** Sat Aug 18, 2018 8:44 pm **Posts:** 127

i am reading minix3.1 text book, the save procedure in the book on page 712 tests
whether the caller enter the kernel for the first time (from user to kernel) or later
(from kernel to kernel).
on line 6638 jmp RETADR-P_STACKBASE(eax)

The RETADR = 24
P_STACKBASE = 0
eax is essentially esp before testing k_reenter for kernel stack switch
so this statement is essentially

jmp 24-0(esp)

how does this work? it looks rather odd to me

Code:

06613
!*===========================================================================*
!* save *
!*===========================================================================*
! Save for protected mode.
! This is much simpler than for 8086 mode, because the stack already points
! into the process table, or has already been switched to the kernel stack.
06620
.align 16
save:
cld ! set direction flag to a known value
pushad ! save "general" registers
o16 push ds ! save ds
o16 push es ! save es
o16 push fs ! save fs
o16 push gs ! save gs
mov dx, ss ! ss is kernel data segment
mov ds, dx ! load rest of kernel segments
mov es, dx ! kernel does not use fs, gs
mov eax, esp ! prepare to return
incb (_k_reenter) ! from -1 if not reentering
jnz set_restart1 ! stack is already kernel stack
mov esp, k_stktop
push _restart ! build return address for int handler
xor ebp, ebp ! for stacktrace
jmp RETADR-P_STACKBASE(eax)
06639
.align 4
set_restart1:
push restart1
jmp RETADR-P_STACKBASE(eax)
06644

Octocontrabass · **Joined:** Mon Mar 25, 2013 7:01 pm **Posts:** 5146

ITchimp wrote:

The RETADR = 24

No, RETADR is 40.

ITchimp · **Joined:** Sat Aug 18, 2018 8:44 pm **Posts:** 127

i figure that out, it is just jmp -40(eax); i didn't take into account of the precedence

nullplan · **Joined:** Wed Aug 30, 2017 8:24 am **Posts:** 1605

No, it is jmp 40(eax) (BTW, that is a weird assembler syntax, like a weird hybrid between AT&T and Intel syntax). The offset is positive. It is reading the return pointer from stack and jumping there.

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minix 3.1's jmp statement problem

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