mirror of
https://github.com/pkivolowitz/asm_book.git
synced 2026-06-21 01:56:47 +08:00
thank you Marc G.
This commit is contained in:
Perry Kivolowitz
parent
fedf9aef57
commit
36e97c78f7
1 changed files with 99 additions and 99 deletions
|
|
@ -131,15 +131,15 @@ Notice the following:
|
|||
Consider this code to sum up the values in an array:
|
||||
|
||||
```c
|
||||
long Sum(long * values, long length) /* 1 */
|
||||
{ /* 2 */
|
||||
long sum = 0; /* 3 */
|
||||
for (long i = 0; i < length; i++) /* 4 */
|
||||
{ /* 5 */
|
||||
sum += values[i]; /* 6 */
|
||||
} /* 7 */
|
||||
return sum; /* 8 */
|
||||
} /* 9 */
|
||||
long Sum(long * values, long length) /* 1 */
|
||||
{ /* 2 */
|
||||
long sum = 0; /* 3 */
|
||||
for (long i = 0; i < length; i++) /* 4 */
|
||||
{ /* 5 */
|
||||
sum += values[i]; /* 6 */
|
||||
} /* 7 */
|
||||
return sum; /* 8 */
|
||||
} /* 9 */
|
||||
```
|
||||
|
||||
We're not going to translate this to assembly language. Instead, we will
|
||||
|
|
@ -152,16 +152,16 @@ fantastically inefficient (in this case).
|
|||
Consider the following code that performs the same function:
|
||||
|
||||
```c
|
||||
long Sum(long * values, long length) /* 1 */
|
||||
{ /* 2 */
|
||||
long sum = 0; /* 3 */
|
||||
long * end = values + length; /* 4 */
|
||||
while (values < end) /* 5 */
|
||||
{ /* 6 */
|
||||
sum += *(values++); /* 7 */
|
||||
} /* 8 */
|
||||
return sum; /* 9 */
|
||||
} /* 10 */
|
||||
long Sum(long * values, long length) /* 1 */
|
||||
{ /* 2 */
|
||||
long sum = 0; /* 3 */
|
||||
long * end = values + length; /* 4 */
|
||||
while (values < end) /* 5 */
|
||||
{ /* 6 */
|
||||
sum += *(values++); /* 7 */
|
||||
} /* 8 */
|
||||
return sum; /* 9 */
|
||||
} /* 10 */
|
||||
```
|
||||
|
||||
Notice we don't use an index variable any longer. Instead, we use the
|
||||
|
|
@ -184,29 +184,29 @@ in both `C` and `C++`. It is *similar in spirit* to this in `C++`:
|
|||
Here is a hand translation of the above `C` code for function `Sum()`:
|
||||
|
||||
```asm
|
||||
.global Sum // 1
|
||||
.text // 2
|
||||
.align 4 // 3
|
||||
// 4
|
||||
// x0 is the pointer to data // 5
|
||||
// x1 is the length and is reused as `end` // 6
|
||||
// x2 is the sum // 7
|
||||
// x3 is the current dereferenced value // 8
|
||||
// 9
|
||||
Sum: // 10
|
||||
mov x2, xzr // 11
|
||||
add x1, x0, x1, lsl 3 // 12
|
||||
b 2f // 13
|
||||
// 14
|
||||
1: ldr x3, [x0], 8 // 15
|
||||
add x2, x2, x3 // 16
|
||||
2: cmp x0, x1 // 17
|
||||
blt 1b // 18
|
||||
// 19
|
||||
mov x0, x2 // 20
|
||||
ret // 21
|
||||
// 22
|
||||
.end // 23
|
||||
.global Sum // 1
|
||||
.text // 2
|
||||
.align 4 // 3
|
||||
|
||||
// x0 is the pointer to data // 5
|
||||
// x1 is the length and is reused as `end` // 6
|
||||
// x2 is the sum // 7
|
||||
// x3 is the current dereferenced value // 8
|
||||
|
||||
Sum: // 10
|
||||
mov x2, xzr // 11
|
||||
add x1, x0, x1, lsl 3 // 12
|
||||
b 2f // 13
|
||||
|
||||
1: ldr x3, [x0], 8 // 15
|
||||
add x2, x2, x3 // 16
|
||||
2: cmp x0, x1 // 17
|
||||
blt 1b // 18
|
||||
|
||||
mov x0, x2 // 20
|
||||
ret // 21
|
||||
|
||||
.end // 23
|
||||
```
|
||||
|
||||
Recall that `Sum(long * values, long length)` means that `x0` has the
|
||||
|
|
@ -371,57 +371,57 @@ You should read the chapter on `struct` found [here](where?).
|
|||
Here is a more elaborate case study. Given this:
|
||||
|
||||
```c
|
||||
#include <stdio.h> /* 1 */
|
||||
/* 2 */
|
||||
struct Person /* 3 */
|
||||
{ /* 4 */
|
||||
char * fname; /* 5 */
|
||||
char * lname; /* 6 */
|
||||
int age; /* 7 */
|
||||
}; /* 8 */
|
||||
/* 9 */
|
||||
extern int rand(); /* 10 */
|
||||
extern struct Person * FindOldestPerson(struct Person *, int); /* 11 */
|
||||
/* 12 */
|
||||
#include <stdio.h> /* 1 */
|
||||
|
||||
struct Person /* 3 */
|
||||
{ /* 4 */
|
||||
char * fname; /* 5 */
|
||||
char * lname; /* 6 */
|
||||
int age; /* 7 */
|
||||
}; /* 8 */
|
||||
|
||||
extern int rand(); /* 10 */
|
||||
extern struct Person * FindOldestPerson(struct Person *, int); /* 11 */
|
||||
|
||||
struct Person * OriginalFindOldestPerson(struct Person * people, int length) /* 13 */
|
||||
{ /* 14 */
|
||||
int oldest_age = 0; /* 15 */
|
||||
struct Person * oldest_ptr = NULL; /* 16 */
|
||||
/* 17 */
|
||||
if (people) /* 18 */
|
||||
{ /* 19 */
|
||||
struct Person * end_ptr = people + length; /* 20 */
|
||||
while (people < end_ptr) /* 21 */
|
||||
{ /* 22 */
|
||||
if (people->age > oldest_age) /* 23 */
|
||||
{ /* 24 */
|
||||
oldest_age = people->age; /* 25 */
|
||||
oldest_ptr = people; /* 26 */
|
||||
} /* 27 */
|
||||
people++; /* 28 */
|
||||
} /* 29 */
|
||||
} /* 30 */
|
||||
return oldest_ptr; /* 31 */
|
||||
} /* 32 */
|
||||
/* 33 */
|
||||
#define LENGTH 20 /* 34 */
|
||||
/* 35 */
|
||||
int main() /* 36 */
|
||||
{ /* 37 */
|
||||
struct Person array[LENGTH]; /* 38 */
|
||||
for (int i = 0; i < LENGTH; i++) /* 39 */
|
||||
{ /* 40 */
|
||||
array[i].age = rand() % 5000; /* 41 */
|
||||
} /* 42 */
|
||||
struct Person * oldest = FindOldestPerson(array, LENGTH); /* 43 */
|
||||
for (int i = 0; i < LENGTH; i++) /* 44 */
|
||||
{ /* 45 */
|
||||
printf("%d", array[i].age); /* 46 */
|
||||
if (oldest == &array[i]) /* 47 */
|
||||
printf("*"); /* 48 */
|
||||
printf("\n"); /* 49 */
|
||||
} /* 50 */
|
||||
} /* 51 */
|
||||
{ /* 14 */
|
||||
int oldest_age = 0; /* 15 */
|
||||
struct Person * oldest_ptr = NULL; /* 16 */
|
||||
|
||||
if (people) /* 18 */
|
||||
{ /* 19 */
|
||||
struct Person * end_ptr = people + length; /* 20 */
|
||||
while (people < end_ptr) /* 21 */
|
||||
{ /* 22 */
|
||||
if (people->age > oldest_age) /* 23 */
|
||||
{ /* 24 */
|
||||
oldest_age = people->age; /* 25 */
|
||||
oldest_ptr = people; /* 26 */
|
||||
} /* 27 */
|
||||
people++; /* 28 */
|
||||
} /* 29 */
|
||||
} /* 30 */
|
||||
return oldest_ptr; /* 31 */
|
||||
} /* 32 */
|
||||
|
||||
#define LENGTH 20 /* 34 */
|
||||
|
||||
int main() /* 36 */
|
||||
{ /* 37 */
|
||||
struct Person array[LENGTH]; /* 38 */
|
||||
for (int i = 0; i < LENGTH; i++) /* 39 */
|
||||
{ /* 40 */
|
||||
array[i].age = rand() % 5000; /* 41 */
|
||||
} /* 42 */
|
||||
struct Person * oldest = FindOldestPerson(array, LENGTH); /* 43 */
|
||||
for (int i = 0; i < LENGTH; i++) /* 44 */
|
||||
{ /* 45 */
|
||||
printf("%d", array[i].age); /* 46 */
|
||||
if (oldest == &array[i]) /* 47 */
|
||||
printf("*"); /* 48 */
|
||||
printf("\n"); /* 49 */
|
||||
} /* 50 */
|
||||
} /* 51 */
|
||||
```
|
||||
|
||||
This program defines a `struct` called `Person`. See `line 3`.
|
||||
|
|
@ -440,9 +440,9 @@ serve only to move the location of the `age` member away from offset 0.
|
|||
same name so that the linker can reconcile the reference to
|
||||
`FindOldestPerson`.
|
||||
|
||||
`OriginalFindOldestPerson` takes a pointer to an instance of `struct Person`. Being a pointer,
|
||||
this can be used as a way of finding just one instance or, as in our case, an array of these
|
||||
`structs`.
|
||||
`OriginalFindOldestPerson` takes a pointer to an instance of `struct
|
||||
Person`. Being a pointer, this can be used as a way of finding just one
|
||||
instance or, as in our case, an array of these `structs`.
|
||||
|
||||
The function finds the largest value in the `age` member using the
|
||||
expected algorithm. It initializes an `oldest_age` found so far with 0
|
||||
|
|
@ -614,7 +614,7 @@ struct must be a multiple of 8. Four wasted bytes are added to the
|
|||
add long*. Here is the instruction:
|
||||
|
||||
```asm
|
||||
smaddl x4, w1, w5, x3 // initialize end_ptr // 19
|
||||
smaddl x4, w1, w5, x3 // initialize end_ptr // 19
|
||||
```
|
||||
|
||||
`w1` (the length) will be multiplied by `w5` (the size of each array
|
||||
|
|
@ -623,7 +623,7 @@ will be placed into `x4`. This assembly language instruction implements
|
|||
this in C:
|
||||
|
||||
```c
|
||||
struct Person * end_ptr = people + length; /* 20 */
|
||||
struct Person * end_ptr = people + length; /* 20 */
|
||||
```
|
||||
|
||||
The compiler itself knows the true length of a `Person` (it is 24). When
|
||||
|
|
@ -656,8 +656,8 @@ resulted in a less than zero, zero, or more than zero result.
|
|||
`Lines 24` and `25` read:
|
||||
|
||||
```asm
|
||||
csel w2, w2, w5, gt // update based on cmp // 24
|
||||
csel x0, x0, x3, gt // update based on cmp // 25
|
||||
csel w2, w2, w5, gt // update based on cmp // 24
|
||||
csel x0, x0, x3, gt // update based on cmp // 25
|
||||
```
|
||||
|
||||
These are identical to this:
|
||||
|
|
|
|||
Loading…
Reference in a new issue