mirror of
https://github.com/pkivolowitz/asm_book.git
synced 2026-06-21 03:46:48 +08:00
beginnings of bit fields
This commit is contained in:
Perry Kivolowitz
parent
b29469a255
commit
dc6ac5a73a
4 changed files with 409 additions and 0 deletions
111
section_2/bitfields/README.md
Normal file
111
section_2/bitfields/README.md
Normal file
|
|
@ -0,0 +1,111 @@
|
|||
# Section 2 / Bit Fields
|
||||
|
||||
## Overview
|
||||
|
||||
Many C and C++ programmers have never seen bit fields.
|
||||
|
||||
Bit fields are a
|
||||
feature of the C and C++ language which completely hide what is often
|
||||
called "bit bashing".
|
||||
|
||||
Bit bashing is the manipulation of individual bits. Bit
|
||||
bashing goes to the very core of the C language. Remember that C is a
|
||||
high level assembly language, as we argue in Section 1 of this book.
|
||||
And C is the (later) language in which Unix was implemented and indeed,
|
||||
C was
|
||||
developed specifically to implement Unix.
|
||||
|
||||
Since an operating system directly
|
||||
interfaces with hardware - the C language grew to have features
|
||||
to aid Unix implementers.
|
||||
|
||||
*With that said, consider this WARNING: the ordering of bits in a bit
|
||||
field is not guaranteed to be the same on different platforms and even
|
||||
between different compilers on the same platform.*
|
||||
|
||||
Bit fields are implemented within a `struct` by appending a colon plus
|
||||
a number after the declaration of integer types.
|
||||
|
||||
For example:
|
||||
|
||||
```c
|
||||
struct BF {
|
||||
unsigned char a : 1;
|
||||
unsigned char b : 2;
|
||||
unsigned char c : 5;
|
||||
};
|
||||
```
|
||||
|
||||
The above declares a `struct` whose size is 1 byte. Members of the `struct`
|
||||
are `a`, `b` and `c` which are 1, 2 and 5 bits in size, respectively.
|
||||
|
||||
## Bit Fields Aren't Just For Hardware
|
||||
|
||||
Consider a data structure for which there will be potentially millions of
|
||||
instances in RAM. Or, perhaps billions of instances on disc. Suppose you
|
||||
need 8 boolean members in every instance. The C++ standard does not
|
||||
define the size of a `bool` instead leaving it to be implementation
|
||||
dependent. Some implementations equate `bool` to `int`, four bytes in
|
||||
length. Some implement `bool` with a `char`, or 1 byte in length.
|
||||
|
||||
Let's assume the smallest case and equate a `bool` with `char`. Our
|
||||
`struct`, for which there may be millions or billions of instances
|
||||
requires 8 `bool` so therefore 8 bytes. Times millions or billions.
|
||||
|
||||
Ouch.
|
||||
|
||||
Bit fields can come to your aid here by using a single bit per boolean
|
||||
value. In the best case, 8 bytes collapse to 1 byte. In a worse case,
|
||||
8 x 4 = 32 bytes collapsed into 1.
|
||||
|
||||
## Without Bit Fields
|
||||
|
||||
Let's assume we're working with a byte that is comprised of three
|
||||
fields layed out as in `struct BF` above. That is, a one, two and
|
||||
five bit field inside one byte.
|
||||
|
||||
Without bit fields, we would have to write this code:
|
||||
|
||||
```c
|
||||
/* Note the absence of defensive programming such as checking
|
||||
to ensure that byte is not null and that bit_number is not
|
||||
too large.
|
||||
*/
|
||||
|
||||
void ClearA(unsigned char * byte) {
|
||||
*byte &= ~1;
|
||||
}
|
||||
|
||||
void SetA(unsigned char * byte) {
|
||||
*byte &= ~1;
|
||||
*byte |= 1;
|
||||
}
|
||||
|
||||
void ClearB(unsigned char * byte) {
|
||||
*byte &= ~6;
|
||||
}
|
||||
|
||||
void SetB(unsigned char * byte, unsigned char value) {
|
||||
value &= 3; // ensures only bits 0 and 1 can be set
|
||||
*byte &= ~6; // clears bits 1 and 2 in byte
|
||||
*byte |= (value << 1); // stores bits 0 and 1 into bits 2 and 3
|
||||
}
|
||||
|
||||
void ClearC(unsigned char * byte) {
|
||||
*byte &= 7; // squashes bits 3 to 7 to 0
|
||||
}
|
||||
|
||||
void SetC(unsigned char * byte, unsigned char value) {
|
||||
value &= 0x1F; // ensures only bits 0 to 4 can be set
|
||||
*byte &= ~(0x1F << 3); // squashes correct bits in byte
|
||||
*byte |= (value << 3); // or's in the bits at the right place
|
||||
}
|
||||
```
|
||||
|
||||
In naive assembly language, these functions would look like this:
|
||||
|
||||
```asm
|
||||
```
|
||||
|
||||
|
||||
|
||||
65
section_2/bitfields/naive.s
Normal file
65
section_2/bitfields/naive.s
Normal file
|
|
@ -0,0 +1,65 @@
|
|||
.global SetA
|
||||
.global SetB
|
||||
.global SetC
|
||||
.global ClearA
|
||||
.global ClearB
|
||||
.global ClearC
|
||||
|
||||
.text
|
||||
.align 2
|
||||
|
||||
ClearA: ldrb w1, [x0]
|
||||
mov w2, 1
|
||||
mvn w2, w2
|
||||
and w1, w1, w2
|
||||
strb w1, [x0]
|
||||
ret
|
||||
|
||||
ClearB: ldrb w1, [x0]
|
||||
mov w2, 6
|
||||
mvn w2, w2
|
||||
and w1, w1, w2
|
||||
strb w1, [x0]
|
||||
ret
|
||||
|
||||
ClearC: ldrb w1, [x0]
|
||||
and w1, w1, 7
|
||||
strb w1, [x0]
|
||||
ret
|
||||
|
||||
SetA: ldrb w1, [x0]
|
||||
mov w2, 1
|
||||
mvn w2, w2
|
||||
and w1, w1, w2
|
||||
orr w1, w1, 1
|
||||
strb w1, [x0]
|
||||
ret
|
||||
|
||||
|
||||
SetB: ldrb w3, [x0]
|
||||
and w1, w1, 3 // value &= 3
|
||||
lsl w1, w1, 1
|
||||
mov w2, 6
|
||||
mvn w2, w2
|
||||
and w3, w3, w2 // B is cleared
|
||||
orr w3, w3, w1
|
||||
strb w3, [x0]
|
||||
ret
|
||||
|
||||
SetC: ldrb w3, [x0]
|
||||
|
||||
mov w2, 0x1F
|
||||
and w1, w1, w2
|
||||
lsl w1, w1, 3
|
||||
|
||||
lsl w2, w2, 3
|
||||
mvn w2, w2
|
||||
and w3, w3, w2
|
||||
|
||||
orr w3, w3, w1
|
||||
strb w3, [x0]
|
||||
ret
|
||||
|
||||
.end
|
||||
|
||||
|
||||
69
section_2/bitfields/test.c
Normal file
69
section_2/bitfields/test.c
Normal file
|
|
@ -0,0 +1,69 @@
|
|||
#include <stdio.h>
|
||||
|
||||
struct BF {
|
||||
unsigned char a : 1;
|
||||
unsigned char b : 2;
|
||||
unsigned char c : 5;
|
||||
};
|
||||
|
||||
unsigned char noBF = 0;
|
||||
|
||||
#undef C
|
||||
#ifdef C
|
||||
/* Note the absence of defensive programming such as checking
|
||||
to ensure that byte is not null and that bit_number is not
|
||||
too large.
|
||||
*/
|
||||
|
||||
void ClearA(unsigned char * byte) {
|
||||
*byte &= ~1;
|
||||
}
|
||||
|
||||
void SetA(unsigned char * byte) {
|
||||
*byte &= ~1;
|
||||
*byte |= 1;
|
||||
}
|
||||
|
||||
void ClearB(unsigned char * byte) {
|
||||
*byte &= ~6;
|
||||
}
|
||||
|
||||
void SetB(unsigned char * byte, unsigned char value) {
|
||||
value &= 3; // ensures only bits 0 and 1 can be set
|
||||
*byte &= ~6; // clears bits 1 and 2 in byte
|
||||
*byte |= (value << 1); // stores bits 0 and 1 into bits 2 and 3
|
||||
}
|
||||
|
||||
void ClearC(unsigned char * byte) {
|
||||
*byte &= 7; // squashes bits 3 to 7 to 0
|
||||
}
|
||||
|
||||
void SetC(unsigned char * byte, unsigned char value) {
|
||||
value &= 0x1F; // ensures only bits 0 to 4 can be set
|
||||
*byte &= ~(0x1F << 3); // squashes correct bits in byte
|
||||
*byte |= (value << 3); // or's in the bits at the right place
|
||||
}
|
||||
#else
|
||||
extern void SetA(unsigned char *);
|
||||
extern void SetB(unsigned char *, unsigned char);
|
||||
extern void SetC(unsigned char *, unsigned char);
|
||||
extern void ClearA(unsigned char *);
|
||||
extern void ClearB(unsigned char *);
|
||||
extern void ClearC(unsigned char *);
|
||||
#endif
|
||||
|
||||
int main() {
|
||||
volatile struct BF bf;
|
||||
|
||||
bf.a = 1;
|
||||
bf.b = 2;
|
||||
bf.c = 3;
|
||||
|
||||
ClearA(&noBF);
|
||||
SetB(&noBF, 1);
|
||||
SetC(&noBF, 7);
|
||||
|
||||
printf("noBF should be 0x3A - value: 0x%X\n", (unsigned int) noBF);
|
||||
printf("bf should be 0x1D - value: 0x%X\n", (unsigned int) *((unsigned char *) &bf));
|
||||
return 0;
|
||||
}
|
||||
164
section_2/bitfields/test.s
Normal file
164
section_2/bitfields/test.s
Normal file
|
|
@ -0,0 +1,164 @@
|
|||
.arch armv8-a
|
||||
.file "test.c"
|
||||
.text
|
||||
.align 2
|
||||
.p2align 3,,7
|
||||
.global Foo
|
||||
.type Foo, %function
|
||||
Foo:
|
||||
.LFB23:
|
||||
.cfi_startproc
|
||||
and w0, w0, 255
|
||||
mvn w0, w0
|
||||
ret
|
||||
.cfi_endproc
|
||||
.LFE23:
|
||||
.size Foo, .-Foo
|
||||
.align 2
|
||||
.p2align 3,,7
|
||||
.global ClearA
|
||||
.type ClearA, %function
|
||||
ClearA:
|
||||
.LFB24:
|
||||
.cfi_startproc
|
||||
ldrb w1, [x0]
|
||||
and w1, w1, -2
|
||||
strb w1, [x0]
|
||||
ret
|
||||
.cfi_endproc
|
||||
.LFE24:
|
||||
.size ClearA, .-ClearA
|
||||
.align 2
|
||||
.p2align 3,,7
|
||||
.global SetA
|
||||
.type SetA, %function
|
||||
SetA:
|
||||
.LFB25:
|
||||
.cfi_startproc
|
||||
ldrb w1, [x0]
|
||||
orr w1, w1, 1
|
||||
strb w1, [x0]
|
||||
ret
|
||||
.cfi_endproc
|
||||
.LFE25:
|
||||
.size SetA, .-SetA
|
||||
.align 2
|
||||
.p2align 3,,7
|
||||
.global ClearB
|
||||
.type ClearB, %function
|
||||
ClearB:
|
||||
.LFB26:
|
||||
.cfi_startproc
|
||||
ldrb w1, [x0]
|
||||
and w1, w1, -7
|
||||
strb w1, [x0]
|
||||
ret
|
||||
.cfi_endproc
|
||||
.LFE26:
|
||||
.size ClearB, .-ClearB
|
||||
.align 2
|
||||
.p2align 3,,7
|
||||
.global SetB
|
||||
.type SetB, %function
|
||||
SetB:
|
||||
.LFB27:
|
||||
.cfi_startproc
|
||||
ldrb w2, [x0]
|
||||
bfi w2, w1, 1, 2
|
||||
strb w2, [x0]
|
||||
ret
|
||||
.cfi_endproc
|
||||
.LFE27:
|
||||
.size SetB, .-SetB
|
||||
.align 2
|
||||
.p2align 3,,7
|
||||
.global ClearC
|
||||
.type ClearC, %function
|
||||
ClearC:
|
||||
.LFB28:
|
||||
.cfi_startproc
|
||||
ldrb w1, [x0]
|
||||
and w1, w1, 7
|
||||
strb w1, [x0]
|
||||
ret
|
||||
.cfi_endproc
|
||||
.LFE28:
|
||||
.size ClearC, .-ClearC
|
||||
.align 2
|
||||
.p2align 3,,7
|
||||
.global SetC
|
||||
.type SetC, %function
|
||||
SetC:
|
||||
.LFB29:
|
||||
.cfi_startproc
|
||||
ldrb w2, [x0]
|
||||
ubfiz w1, w1, 3, 5
|
||||
and w2, w2, 7
|
||||
orr w2, w2, w1
|
||||
strb w2, [x0]
|
||||
ret
|
||||
.cfi_endproc
|
||||
.LFE29:
|
||||
.size SetC, .-SetC
|
||||
.section .rodata.str1.8,"aMS",@progbits,1
|
||||
.align 3
|
||||
.LC0:
|
||||
.string "noBF should be 0x3A - value: 0x%X\n"
|
||||
.align 3
|
||||
.LC1:
|
||||
.string "bf should be 0x1D - value: 0x%X\n"
|
||||
.section .text.startup,"ax",@progbits
|
||||
.align 2
|
||||
.p2align 3,,7
|
||||
.global main
|
||||
.type main, %function
|
||||
main:
|
||||
.LFB30:
|
||||
.cfi_startproc
|
||||
stp x29, x30, [sp, -32]!
|
||||
.cfi_def_cfa_offset 32
|
||||
.cfi_offset 29, -32
|
||||
.cfi_offset 30, -24
|
||||
mov w5, 2
|
||||
mov w4, 3
|
||||
mov x29, sp
|
||||
ldrb w0, [sp, 24]
|
||||
adrp x2, .LANCHOR0
|
||||
adrp x1, .LC0
|
||||
add x1, x1, :lo12:.LC0
|
||||
orr w0, w0, 1
|
||||
strb w0, [sp, 24]
|
||||
mov w0, 58
|
||||
strb w0, [x2, #:lo12:.LANCHOR0]
|
||||
ldrb w3, [sp, 24]
|
||||
mov w2, 58
|
||||
mov w0, 1
|
||||
bfi w3, w5, 1, 2
|
||||
strb w3, [sp, 24]
|
||||
ldrb w3, [sp, 24]
|
||||
bfi w3, w4, 3, 5
|
||||
strb w3, [sp, 24]
|
||||
bl __printf_chk
|
||||
ldrb w2, [sp, 24]
|
||||
adrp x1, .LC1
|
||||
mov w0, 1
|
||||
add x1, x1, :lo12:.LC1
|
||||
bl __printf_chk
|
||||
mov w0, 0
|
||||
ldp x29, x30, [sp], 32
|
||||
.cfi_restore 30
|
||||
.cfi_restore 29
|
||||
.cfi_def_cfa_offset 0
|
||||
ret
|
||||
.cfi_endproc
|
||||
.LFE30:
|
||||
.size main, .-main
|
||||
.global noBF
|
||||
.bss
|
||||
.set .LANCHOR0,. + 0
|
||||
.type noBF, %object
|
||||
.size noBF, 1
|
||||
noBF:
|
||||
.zero 1
|
||||
.ident "GCC: (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0"
|
||||
.section .note.GNU-stack,"",@progbits
|
||||
Loading…
Reference in a new issue