Question

Write a procedure BCD to convert a hexadecimal number in RA into its BCD (binary-coded decimal) representation and place it in RB. The return address should be placed in RF. For example, if RA=0x0024 before the call, RB=0x0036 after the all.

// can someone tell me how to write that//

Answer 1

0x0024 is 2*16+4=36, which is what is required in RB 0x0036.
Basically, the hex value is converted into decimal, and each decimal digit is stored as a byte in RB.

The algorithm to do this is to divide the hex number by 10 (decimal), store the integer quotient in a register, and the remainder is the first byte in RB.
Repeat the process, i.e. divide the quotient by 10(dec.). However, the remainder must be shifted 8 bits to the left before adding to RB to get the next digit.

Repeat, if necessary, until the quotient is zero.

Example:
RA = 0x0024
RB = 0x0000
Divide by 0x000A to get
RA = 0x0003 (quotient)
RC = 0x0006 (remainder)
Add rb,rc
RB = 0x0006

if RA≠0, Repeat division
RA = 0x0000
RC = 0x0003 (remainder)
shift left 8 bits
RC = 0x0030
add rb,rc
RB = 0x0036

etc.

You will have to do some register management to put the quotient and remainder in the right places.