C语言 intel架构处理器下利用gcc内联汇编 xlat 指令 实现转换1字节十六进制数字到十进制

最近发现英特尔处理器有一个从bx指向的内存索引一个字节替换到al的xlat指令，遂编写此简单的转换程序以作练习。由于一次只能查找一字节，而紧凑bcd码最大占用2字节，因此需要用xlat索引2次。

#include <stdio.h>
#include <stdint.h>
#include <string.h>

static uint8_t hex2decl[] = 
    {
    
     0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09
    , 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19
    , 0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29
    , 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39
    , 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49
    , 0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59
    , 0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69
    , 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79
    , 0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89
    , 0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99
    , 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09
    , 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19
    , 0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29
    , 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39
    , 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49
    , 0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59
    , 0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69
    , 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79
    , 0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89
    , 0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99
    , 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09
    , 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19
    , 0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29
    , 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39
    , 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49
    , 0x50, 0x51, 0x52, 0x53, 0x54, 0x55};

static uint8_t hex2dech[] =
    {
    
     0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x10
    , 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20
    , 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20
    , 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20
    , 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20
    , 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20
    , 0x20, 0x20, 0x20, 0x20, 0x20, 0x20};

int turnbcd(const char* u16ptr) {
    
    
    register int a = ((0xff&u16ptr[1])|(0xff00&(((int)u16ptr[0])<<8)))-0x3030;
    __asm__ (
        "cmpb $10, %%ah\n\t"
        "cmovaw %4, %%cx\n\t"
        "subb %%cl, %%ah\n\t"
        "cmpb $10, %%al\n\t"
        "cmovaw %5, %%cx\n\t"
        "subb %%ch, %%al\n\t"
        "movb $4, %%cl\n\t"
        "shlb %%cl, %%ah\n\t"
        "orb  %%ah, %%al\n\t"
        "movb %%al, %%ah\n\t"
        "xlat\n\t"
        "xchgb %%al, %%ah\n\t"
        "mov %6, %2\n\t"
        "xlat\n\t"
        "movl %%eax, %%ebx\n\t"
        "shll %%cl, %%ebx\n\t"
        "andl $0x000f0f00, %%ebx\n\t"
        "andl $0x0000000f, %%eax\n\t"
        "orl  %%ebx, %%eax"
        : "=a" (a)
        : "0" (a), "b" (hex2dech), "c" (0), "r" ((uint16_t)0x0027), "r" ((uint16_t)0x2700), "r" (hex2decl)
    );
    return a;
}

static int ishexchar(char c) {
    
    
    return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f');
}

static void printbcd(uint32_t r) {
    
    
    r += 0x303030;
    char buf[4];
    buf[0] = (char)((r & 0x00ff0000) >> 16);
    buf[1] = (char)((r & 0x0000ff00) >> 8);
    buf[2] = (char)r;
    buf[3] = 0;
    puts(buf);
    return;
}

// 内联汇编转换1字节16进制数字到十进制
// 输入: 1字节16进制数字 (ff)
// 输出: 10进制数字 (256)
int main(int argc, char** argv) {
    
    
    if (argc != 2) {
    
    
        puts("usage: ff");
        return -1;
    }
    int n = strlen(argv[1]);
    if (n != 2 || !ishexchar(argv[1][0]) || !ishexchar(argv[1][1])) {
    
    
        puts("usage: ff");
        return -2;
    }
    printbcd(turnbcd(argv[1]));
}