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分享今天看到个奇怪的代码，求科普
今天看到一个奇怪的代码。
unsigned long a;//这里是我找到的变量定义。
/*下面是我，感觉奇怪的地方*/
a >>= 32;
a <<= 13;
/*奇怪的第一个点, a>>=32;
我拆分出来，a= a>>32;这种结果我计算机出是0;
奇怪第二点，a <<=13;如果前面是0，那紧跟这一句，不是多此一举。
long 做为关键字，难道还可以使用
自定义，宏替换之类的手法，把long long 改成long.
何况书还是9几年写的，难道那个时候64位pc机就以经，普及了，
*/
...全文
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mymtom 2015-09-30
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楼上真强大啊！
ALPHA处理器（1993），20多年了，最早的64位架构之一， Linux用的LP64，所以long是64位。
ALPHA处理器（1993），20多年了，最早的64位架构之一， Linux用的LP64，所以long是64位。
logiciel 2015-09-30
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Linux/arch/alpha/boot/main.c
1 /*
2 * arch/alpha/boot/main.c
3 *
4 * Copyright (C) 1994, 1995 Linus Torvalds
5 *
6 * This file is the bootloader for the Linux/AXP kernel
7 */
8 #include <linux/kernel.h>
9 #include <linux/slab.h>
10 #include <linux/string.h>
11 #include <generated/utsrelease.h>
12 #include <linux/mm.h>
13
14 #include <asm/console.h>
15 #include <asm/hwrpb.h>
16 #include <asm/pgtable.h>
17
18 #include <stdarg.h>
19
20 #include "ksize.h"
21
22 extern unsigned long switch_to_osf_pal(unsigned long nr,
23 struct pcb_struct * pcb_va, struct pcb_struct * pcb_pa,
24 unsigned long *vptb);
25 struct hwrpb_struct *hwrpb = INIT_HWRPB;
26 static struct pcb_struct pcb_va[1];
27
28 /*
29 * Find a physical address of a virtual object..
30 *
31 * This is easy using the virtual page table address.
32 */
33
34 static inline void *
35 find_pa(unsigned long *vptb, void *ptr)
36 {
37 unsigned long address = (unsigned long) ptr;
38 unsigned long result;
39
40 result = vptb[address >> 13];
41 result >>= 32;
42 result <<= 13;
43 result |= address & 0x1fff;
44 return (void *) result;
45 }
46
47 /*
48 * This function moves into OSF/1 pal-code, and has a temporary
49 * PCB for that. The kernel proper should replace this PCB with
50 * the real one as soon as possible.
51 *
52 * The page table muckery in here depends on the fact that the boot
53 * code has the L1 page table identity-map itself in the second PTE
54 * in the L1 page table. Thus the L1-page is virtually addressable
55 * itself (through three levels) at virtual address 0x200802000.
56 */
57
58 #define VPTB ((unsigned long *) 0x200000000)
59 #define L1 ((unsigned long *) 0x200802000)
60
61 void
62 pal_init(void)
63 {
64 unsigned long i, rev;
65 struct percpu_struct * percpu;
66 struct pcb_struct * pcb_pa;
67
68 /* Create the dummy PCB. */
69 pcb_va->ksp = 0;
70 pcb_va->usp = 0;
71 pcb_va->ptbr = L1[1] >> 32;
72 pcb_va->asn = 0;
73 pcb_va->pcc = 0;
74 pcb_va->unique = 0;
75 pcb_va->flags = 1;
76 pcb_va->res1 = 0;
77 pcb_va->res2 = 0;
78 pcb_pa = find_pa(VPTB, pcb_va);
79
80 /*
81 * a0 = 2 (OSF)
82 * a1 = return address, but we give the asm the vaddr of the PCB
83 * a2 = physical addr of PCB
84 * a3 = new virtual page table pointer
85 * a4 = KSP (but the asm sets it)
86 */
87 srm_printk("Switching to OSF PAL-code .. ");
88
89 i = switch_to_osf_pal(2, pcb_va, pcb_pa, VPTB);
90 if (i) {
91 srm_printk("failed, code %ld\n", i);
92 __halt();
93 }
94
95 percpu = (struct percpu_struct *)
96 (INIT_HWRPB->processor_offset + (unsigned long) INIT_HWRPB);
97 rev = percpu->pal_revision = percpu->palcode_avail[2];
98
99 srm_printk("Ok (rev %lx)\n", rev);
100
101 tbia(); /* do it directly in case we are SMP */
102 }
103
104 static inline long openboot(void)
105 {
106 char bootdev[256];
107 long result;
108
109 result = callback_getenv(ENV_BOOTED_DEV, bootdev, 255);
110 if (result < 0)
111 return result;
112 return callback_open(bootdev, result & 255);
113 }
114
115 static inline long close(long dev)
116 {
117 return callback_close(dev);
118 }
119
120 static inline long load(long dev, unsigned long addr, unsigned long count)
121 {
122 char bootfile[256];
123 extern char _end;
124 long result, boot_size = &_end - (char *) BOOT_ADDR;
125
126 result = callback_getenv(ENV_BOOTED_FILE, bootfile, 255);
127 if (result < 0)
128 return result;
129 result &= 255;
130 bootfile[result] = '\0';
131 if (result)
132 srm_printk("Boot file specification (%s) not implemented\n",
133 bootfile);
134 return callback_read(dev, count, (void *)addr, boot_size/512 + 1);
135 }
136
137 /*
138 * Start the kernel.
139 */
140 static void runkernel(void)
141 {
142 __asm__ __volatile__(
143 "bis %1,%1,$30\n\t"
144 "bis %0,%0,$26\n\t"
145 "ret ($26)"
146 : /* no outputs: it doesn't even return */
147 : "r" (START_ADDR),
148 "r" (PAGE_SIZE + INIT_STACK));
149 }
150
151 void start_kernel(void)
152 {
153 long i;
154 long dev;
155 int nbytes;
156 char envval[256];
157
158 srm_printk("Linux/AXP bootloader for Linux " UTS_RELEASE "\n");
159 if (INIT_HWRPB->pagesize != 8192) {
160 srm_printk("Expected 8kB pages, got %ldkB\n", INIT_HWRPB->pagesize >> 10);
161 return;
162 }
163 pal_init();
164 dev = openboot();
165 if (dev < 0) {
166 srm_printk("Unable to open boot device: %016lx\n", dev);
167 return;
168 }
169 dev &= 0xffffffff;
170 srm_printk("Loading vmlinux ...");
171 i = load(dev, START_ADDR, KERNEL_SIZE);
172 close(dev);
173 if (i != KERNEL_SIZE) {
174 srm_printk("Failed (%lx)\n", i);
175 return;
176 }
177
178 nbytes = callback_getenv(ENV_BOOTED_OSFLAGS, envval, sizeof(envval));
179 if (nbytes < 0) {
180 nbytes = 0;
181 }
182 envval[nbytes] = '\0';
183 strcpy((char*)ZERO_PGE, envval);
184
185 srm_printk(" Ok\nNow booting the kernel\n");
186 runkernel();
187 for (i = 0 ; i < 0x100000000 ; i++)
188 /* nothing */;
189 __halt();
190 }
191
bluewanderer 2015-09-29
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首先那会儿比如N64虽然比较失败也卖出去不少来的。然后long到底多少位完全看写编译器的人的心情，现在Win64上long也还是32位。最后不知道你这本书的上下文。
mewiteor 2015-09-29
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fly_dragon_fly 2015-09-29
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书中程序用在什么地方
mujiok2003 2015-09-29
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不要假设long 是32位的
yajuie 2015-09-29
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书是，从宿舍找到的，残破不堪。没几段完整的代码，刚好不多的完整代码中有这么段完整函数，函数完整如下:
void* find_pa(unsigned long *vptb,void *ptr)
{
unsigned long address = (unsigned long) ptr;
unsigned long result;
result = vptb[address >> 32];
result >>= 32;
result <<=13;
result |= address & 0x1fff;
return (void *)result;
}
yajuie 2015-09-28
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求各路大神，指点一下，这到底在什么情况下会写成这样，因为书上这种代码以出现了好几遍了，基本上可以排出bug的可能，
