Multiplication table in VMLab / AVR

Question

I am trying to figure out this question. I posted my code below. It doesn't work properly. it seems like its not multiplying the 2 least significant nibbles. I don't know AVR very well.

Write AVR that generates a multiplication table for SRAM addresses 0x0100 to 0x01FF. The value at each address is the product of the two least significant nibbles of the address. For example, at address 0x0123, the multiplicand is 3 and the multiplier is 2. calculate the product (6 in this case) and store it at address 0x0123. The answer should be about 10-12 lines of code with a loop

    .include "C:\VMLAB\include\m168def.inc"
    ldi r27, 0x01
    ldi r26, 0x00
    ldi r30, 0xff
    main:
    mov r16, r26
    andi r16,0x0f
    mov r17,r27
    andi r17,0xf0
    swap r17
    mul  r17, r16
    st x+, r16
    dec r30
    brne main

Answer 1

According to the manual, the MUL instruction stores its result in R0 and R1. So you need to read R0 after MUL, not R16.

Answer 2

You have three errors in your code:

1. mul stores results to R1:R0 pair registers
2. if use X as the index register and content mark symbolically as ABCD then CD is located in XL register, not in XH (r27). AB is constant (01)
3. you loops only 255 times not 256

    ldi xh, 0x01
    ldi xl, 0x00
    ldi r30, 0x00
    main:
    mov r16, xl    ;put D to R16
    andi r16,0x0f
    mov r17, xl    ;put C to R17
    andi r17,0xf0
    swap r17
    mul  r17, r16  ;multiply C x D
    st x+, r0      ;store result low byte to memory
    dec r30        ;repeat 256 times
    brne main