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thank you Marc G.

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Perry Kivolowitz 2022-12-26 13:25:00 -06:00
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section_1/regs/ldr.md
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@ -131,15 +131,15 @@ Notice the following:
Consider this code to sum up the values in an array: Consider this code to sum up the values in an array:
```c ```c
long Sum(long * values, long length) /* 1 */ long Sum(long * values, long length) /* 1 */
{ /* 2 */ { /* 2 */
long sum = 0; /* 3 */ long sum = 0; /* 3 */
for (long i = 0; i < length; i++) /* 4 */ for (long i = 0; i < length; i++) /* 4 */
{ /* 5 */ { /* 5 */
sum += values[i]; /* 6 */ sum += values[i]; /* 6 */
} /* 7 */ } /* 7 */
return sum; /* 8 */ return sum; /* 8 */
} /* 9 */ } /* 9 */
``` ```
We're not going to translate this to assembly language. Instead, we will 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: Consider the following code that performs the same function:
```c ```c
long Sum(long * values, long length) /* 1 */ long Sum(long * values, long length) /* 1 */
{ /* 2 */ { /* 2 */
long sum = 0; /* 3 */ long sum = 0; /* 3 */
long * end = values + length; /* 4 */ long * end = values + length; /* 4 */
while (values < end) /* 5 */ while (values < end) /* 5 */
{ /* 6 */ { /* 6 */
sum += *(values++); /* 7 */ sum += *(values++); /* 7 */
} /* 8 */ } /* 8 */
return sum; /* 9 */ return sum; /* 9 */
} /* 10 */ } /* 10 */
``` ```
Notice we don't use an index variable any longer. Instead, we use the 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()`: Here is a hand translation of the above `C` code for function `Sum()`:
```asm ```asm
.global Sum // 1 .global Sum // 1
.text // 2 .text // 2
.align 4 // 3 .align 4 // 3
// 4
// x0 is the pointer to data // 5 // x0 is the pointer to data // 5
// x1 is the length and is reused as `end` // 6 // x1 is the length and is reused as `end` // 6
// x2 is the sum // 7 // x2 is the sum // 7
// x3 is the current dereferenced value // 8 // x3 is the current dereferenced value // 8
// 9
Sum: // 10 Sum: // 10
mov x2, xzr // 11 mov x2, xzr // 11
add x1, x0, x1, lsl 3 // 12 add x1, x0, x1, lsl 3 // 12
b 2f // 13 b 2f // 13
// 14
1: ldr x3, [x0], 8 // 15 1: ldr x3, [x0], 8 // 15
add x2, x2, x3 // 16 add x2, x2, x3 // 16
2: cmp x0, x1 // 17 2: cmp x0, x1 // 17
blt 1b // 18 blt 1b // 18
// 19
mov x0, x2 // 20 mov x0, x2 // 20
ret // 21 ret // 21
// 22
.end // 23 .end // 23
``` ```
Recall that `Sum(long * values, long length)` means that `x0` has the 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: Here is a more elaborate case study. Given this:
```c ```c
#include <stdio.h> /* 1 */ #include <stdio.h> /* 1 */
/* 2 */
struct Person /* 3 */ struct Person /* 3 */
{ /* 4 */ { /* 4 */
char * fname; /* 5 */ char * fname; /* 5 */
char * lname; /* 6 */ char * lname; /* 6 */
int age; /* 7 */ int age; /* 7 */
}; /* 8 */ }; /* 8 */
/* 9 */
extern int rand(); /* 10 */ extern int rand(); /* 10 */
extern struct Person * FindOldestPerson(struct Person *, int); /* 11 */ extern struct Person * FindOldestPerson(struct Person *, int); /* 11 */
/* 12 */
struct Person * OriginalFindOldestPerson(struct Person * people, int length) /* 13 */ struct Person * OriginalFindOldestPerson(struct Person * people, int length) /* 13 */
{ /* 14 */ { /* 14 */
int oldest_age = 0; /* 15 */ int oldest_age = 0; /* 15 */
struct Person * oldest_ptr = NULL; /* 16 */ struct Person * oldest_ptr = NULL; /* 16 */
/* 17 */
if (people) /* 18 */ if (people) /* 18 */
{ /* 19 */ { /* 19 */
struct Person * end_ptr = people + length; /* 20 */ struct Person * end_ptr = people + length; /* 20 */
while (people < end_ptr) /* 21 */ while (people < end_ptr) /* 21 */
{ /* 22 */ { /* 22 */
if (people->age > oldest_age) /* 23 */ if (people->age > oldest_age) /* 23 */
{ /* 24 */ { /* 24 */
oldest_age = people->age; /* 25 */ oldest_age = people->age; /* 25 */
oldest_ptr = people; /* 26 */ oldest_ptr = people; /* 26 */
} /* 27 */ } /* 27 */
people++; /* 28 */ people++; /* 28 */
} /* 29 */ } /* 29 */
} /* 30 */ } /* 30 */
return oldest_ptr; /* 31 */ return oldest_ptr; /* 31 */
} /* 32 */ } /* 32 */
/* 33 */
#define LENGTH 20 /* 34 */ #define LENGTH 20 /* 34 */
/* 35 */
int main() /* 36 */ int main() /* 36 */
{ /* 37 */ { /* 37 */
struct Person array[LENGTH]; /* 38 */ struct Person array[LENGTH]; /* 38 */
for (int i = 0; i < LENGTH; i++) /* 39 */ for (int i = 0; i < LENGTH; i++) /* 39 */
{ /* 40 */ { /* 40 */
array[i].age = rand() % 5000; /* 41 */ array[i].age = rand() % 5000; /* 41 */
} /* 42 */ } /* 42 */
struct Person * oldest = FindOldestPerson(array, LENGTH); /* 43 */ struct Person * oldest = FindOldestPerson(array, LENGTH); /* 43 */
for (int i = 0; i < LENGTH; i++) /* 44 */ for (int i = 0; i < LENGTH; i++) /* 44 */
{ /* 45 */ { /* 45 */
printf("%d", array[i].age); /* 46 */ printf("%d", array[i].age); /* 46 */
if (oldest == &array[i]) /* 47 */ if (oldest == &array[i]) /* 47 */
printf("*"); /* 48 */ printf("*"); /* 48 */
printf("\n"); /* 49 */ printf("\n"); /* 49 */
} /* 50 */ } /* 50 */
} /* 51 */ } /* 51 */
``` ```
This program defines a `struct` called `Person`. See `line 3`. 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 same name so that the linker can reconcile the reference to
`FindOldestPerson`. `FindOldestPerson`.
`OriginalFindOldestPerson` takes a pointer to an instance of `struct Person`. Being a pointer, `OriginalFindOldestPerson` takes a pointer to an instance of `struct
this can be used as a way of finding just one instance or, as in our case, an array of these Person`. Being a pointer, this can be used as a way of finding just one
`structs`. instance or, as in our case, an array of these `structs`.
The function finds the largest value in the `age` member using the The function finds the largest value in the `age` member using the
expected algorithm. It initializes an `oldest_age` found so far with 0 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: add long*. Here is the instruction:
```asm ```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 `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: this in C:
```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 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: `Lines 24` and `25` read:
```asm ```asm
csel w2, w2, w5, gt // update based on cmp // 24 csel w2, w2, w5, gt // update based on cmp // 24
csel x0, x0, x3, gt // update based on cmp // 25 csel x0, x0, x3, gt // update based on cmp // 25
``` ```
These are identical to this: These are identical to this: