user-0 提问: 今天看到个奇怪的代码，求科普

回复:
user: user-1
content: https://en.wikipedia.org/wiki/DEC_Alpha https://en.wikipedia.org/wiki/64-bit_computing

user: user-1
content: 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。

user: user-2
content: 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

user: user-3
content: 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。

user: user-4
content: Integer types

user: user-5
content: 书中程序用在什么地方

user: user-6
content: 不要假设long 是32位的

user: user-0
content: 引用 3 楼 fly_dragon_fly的回复:书中程序用在什么地方 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; }

user: user-0
content: 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，