I am working on this tiny operating system called Pintos.I am a bignner to assembly so i was in desprate need of a help.Pintos puts the following assembly file inorder to do the following two things
1. to define 256 fragments of code,named 'intr00_stub' through
'intrff_stub'
2. puts the address of each of these functions in the correct spot in 'intr_stubs', an array of function pointers.

My quiestion is that how are this two things happening if any one cld comment line by line or point me to a document that could do the explainin??
the file is
intr_stubs.S

#include "threads/loader.h"

.text

/* Main interrupt entry point.

An internal or external interrupt starts in one of the
intrNN_stub routines, which push the `struct intr_frame'
frame_pointer, error_code, and vec_no members on the stack,
then jump here.

We save the rest of the `struct intr_frame' members to the
stack, set up some registers as needed by the kernel, and then
call intr_handler(), which actually handles the interrupt.

We "fall through" to intr_exit to return from the interrupt.
*/
.func intr_entry
intr_entry:
/* Save caller's registers. */
pushl %ds
pushl %es
pushl %fs
pushl %gs
pushal

/* Set up kernel environment. */
cld /* String instructions go upward. */
mov $SEL_KDSEG, %eax /* Initialize segment registers. */
mov %eax, %ds
mov %eax, %es
leal 56(%esp), %ebp /* Set up frame pointer. */

/* Call interrupt handler. */
pushl %esp
.globl intr_handler
call intr_handler
addl $4, %esp
.endfunc

/* Interrupt exit.

Restores the caller's registers, discards extra data on the
stack, and returns to the caller.

This is a separate function because it is called directly when
we launch a new user process (see start_process() in
userprog/process.c). */
.globl intr_exit
.func intr_exit
intr_exit:
/* Restore caller's registers. */
popal
popl %gs
popl %fs
popl %es
popl %ds

/* Discard `struct intr_frame' vec_no, error_code,
frame_pointer members. */
addl $12, %esp

/* Return to caller. */
iret
.endfunc

/* Interrupt stubs.

This defines 256 fragments of code, named `intr00_stub'
through `intrff_stub', each of which is used as the entry
point for the corresponding interrupt vector. It also puts
the address of each of these functions in the correct spot in
`intr_stubs', an array of function pointers.

Most of the stubs do this:

1. Push %ebp on the stack (frame_pointer in `struct intr_frame').

2. Push 0 on the stack (error_code).

3. Push the interrupt number on the stack (vec_no).

The CPU pushes an extra "error code" on the stack for a few
interrupts. Because we want %ebp to be where the error code
is, we follow a different path:

1. Push a duplicate copy of the error code on the stack.

2. Replace the original copy of the error code by %ebp.

3. Push the interrupt number on the stack. */

.data
.globl intr_stubs
intr_stubs:

/* This implements steps 1 and 2, described above, in the common
case where we just push a 0 error code. */
#define zero \
pushl %ebp; \
pushl $0

/* This implements steps 1 and 2, described above, in the case
where the CPU already pushed an error code. */
#define REAL \
pushl (%esp); \
movl %ebp, 4(%esp)

/* Emits a stub for interrupt vector NUMBER.
TYPE is `zero', for the case where we push a 0 error code,
or `REAL', if the CPU pushes an error code for us. */
#define STUB(NUMBER, TYPE) \
.text; \
.func intr##NUMBER##_stub; \
intr##NUMBER##_stub: \
TYPE; \
push $0x##NUMBER; \
jmp intr_entry; \
.endfunc; \
\
.data; \
.long intr##NUMBER##_stub;

/* All the stubs. */
STUB(00, zero) STUB(01, zero) STUB(02, zero) STUB(03, zero)
STUB(04, zero) STUB(05, zero) STUB(06, zero) STUB(07, zero)
STUB(08, REAL) STUB(09, zero) STUB(0a, REAL) STUB(0b, REAL)
STUB(0c, zero) STUB(0d, REAL) STUB(0e, REAL) STUB(0f, zero)

STUB(10, zero) STUB(11, REAL) STUB(12, zero) STUB(13, zero)
STUB(14, zero) STUB(15, zero) STUB(16, zero) STUB(17, zero)
STUB(18, REAL) STUB(19, zero) STUB(1a, REAL) STUB(1b, REAL)
STUB(1c, zero) STUB(1d, REAL) STUB(1e, REAL) STUB(1f, zero)

STUB(20, zero) STUB(21, zero) STUB(22, zero) STUB(23, zero)
STUB(24, zero) STUB(25, zero) STUB(26, zero) STUB(27, zero)
STUB(28, zero) STUB(29, zero) STUB(2a, zero) STUB(2b, zero)
STUB(2c, zero) STUB(2d, zero) STUB(2e, zero) STUB(2f, zero)

STUB(30, zero) STUB(31, zero) STUB(32, zero) STUB(33, zero)
STUB(34, zero) STUB(35, zero) STUB(36, zero) STUB(37, zero)
STUB(38, zero) STUB(39, zero) STUB(3a, zero) STUB(3b, zero)
STUB(3c, zero) STUB(3d, zero) STUB(3e, zero) STUB(3f, zero)

STUB(40, zero) STUB(41, zero) STUB(42, zero) STUB(43, zero)
STUB(44, zero) STUB(45, zero) STUB(46, zero) STUB(47, zero)
STUB(48, zero) STUB(49, zero) STUB(4a, zero) STUB(4b, zero)
STUB(4c, zero) STUB(4d, zero) STUB(4e, zero) STUB(4f, zero)

STUB(50, zero) STUB(51, zero) STUB(52, zero) STUB(53, zero)
STUB(54, zero) STUB(55, zero) STUB(56, zero) STUB(57, zero)
STUB(58, zero) STUB(59, zero) STUB(5a, zero) STUB(5b, zero)
STUB(5c, zero) STUB(5d, zero) STUB(5e, zero) STUB(5f, zero)

STUB(60, zero) STUB(61, zero) STUB(62, zero) STUB(63, zero)
STUB(64, zero) STUB(65, zero) STUB(66, zero) STUB(67, zero)
STUB(68, zero) STUB(69, zero) STUB(6a, zero) STUB(6b, zero)
STUB(6c, zero) STUB(6d, zero) STUB(6e, zero) STUB(6f, zero)

STUB(70, zero) STUB(71, zero) STUB(72, zero) STUB(73, zero)
STUB(74, zero) STUB(75, zero) STUB(76, zero) STUB(77, zero)
STUB(78, zero) STUB(79, zero) STUB(7a, zero) STUB(7b, zero)
STUB(7c, zero) STUB(7d, zero) STUB(7e, zero) STUB(7f, zero)

STUB(80, zero) STUB(81, zero) STUB(82, zero) STUB(83, zero)
STUB(84, zero) STUB(85, zero) STUB(86, zero) STUB(87, zero)
STUB(88, zero) STUB(89, zero) STUB(8a, zero) STUB(8b, zero)
STUB(8c, zero) STUB(8d, zero) STUB(8e, zero) STUB(8f, zero)

STUB(90, zero) STUB(91, zero) STUB(92, zero) STUB(93, zero)
STUB(94, zero) STUB(95, zero) STUB(96, zero) STUB(97, zero)
STUB(98, zero) STUB(99, zero) STUB(9a, zero) STUB(9b, zero)
STUB(9c, zero) STUB(9d, zero) STUB(9e, zero) STUB(9f, zero)

dupe reported.

Thu Jul 28, 2016
Ack. They closed the 'other' thread.

Carry on...