BASIC sounds that way:

ax = hexval a$
ax = decimal
a$ = hex string


; ##### AX = hexval a$
mov dptr,#A_ REMARK A_ = A$
mov r0,dpl
mov r1,dph
lcall valhex
mov dptr,#AX
movx @dptr,a

.ZFFFF1:
ljmp .ZFFFF1


valhex:
mov r3,#0
mov r2,#2
valhex2:
movx a,@dptr
inc dptr
clr c
subb a,#65
jnc *+6
add a,#17
sjmp *+4
add a,#10
xch a,r3
swap a
orl a,r3
mov r3,a
djnz r2,valhex2
ret