Maybe start with C, passing this through a compiler:

unsigned char data a[3];
extern void out_a(void);

void doit(unsigned char c)
{
    a[0] = '0' + c/100;
    a[1] = '0' + (c%100)/10;
    a[2] = '0' + (c%10);

    out_a();
}

 

Using SDCC as compiler will get you here:

_doit:
        mov     r2,dpl
;       bin2ascii.c:6: a[0] = '0' + c/100;
        mov     b,#0x64
        mov     a,r2
        div     ab
        add     a,#0x30
        mov     _a,a
;       bin2ascii.c:7: a[1] = '0' + (c%100)/10;
        mov     b,#0x64
        mov     a,r2
        div     ab
        mov     a,b
        mov     b,#0x0A
        div     ab
        add     a,#0x30
        mov     (_a + 0x0001),a
;       bin2ascii.c:8: a[2] = '0' + (c%10);
        mov     b,#0x0A
        mov     a,r2
        div     ab
        mov     a,b
        add     a,#0x30
        mov     (_a + 0x0002),a
;       bin2ascii.c:10: out_a();
        ljmp    _out_a

 

Especially if your knowledge of 8051 assembly code is (still) low the compiler generated code is a good start (you can strip some cycles and bytes, although that likely wont matter on a LCD:)