#include <reg51.h>

typedef unsigned char uint8_t;

// Max 2/10: 0x99. Max signed 2/10: 0x79.
uint8_t ib2d( uint8_t x ) {
    uint8_t mask = 0;
    if( (char)x < 0 ) {
        x *= -1;
        mask = 0x80;
    }
    return ((x/10)<<4)|(x%10)|mask;
}

uint8_t id2b( uint8_t x ) {
    // Convert 2/10 signed num to bin considering sign.
    return (((x>>4)&0x07)*10+(x&0x0f))*( x&0x80 ? -1 : 1);
}

// Input:  P0.
// Output: P1.
main( ) {
    uint8_t num = id2b(P0);
    printf("%x %x\n", num, (uint8_t)-19);
    P1 = ib2d(num);
    // For testing.
    P2 = num;
    while(1);
}