# Beginners introduction to the assembly language of Atmel AVR by Schmidt G.

By Schmidt G.

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Additional info for Beginners introduction to the assembly language of Atmel AVR microprocessors

Sample text

If it is a one, you add the number in the table to the result by reading the numbers from the table using LPM. This is more complicated to program and a little bit slower than the above method. A third method is to calculate the table value, starting with 000001, by adding this BCD with itself, each time after you have shifted a bit from the binary to the right and added the BCD. Many methods, much to optimize here. Multiplication Multiplication of binary numbers is explained here. Decimal multiplication In order to multiply two 8-bit-binaries we remind ourselves, how this is done with decimal numbers: 1234 * 567 = ?

The lower 6 should only be added if the lower nibble either overflows to the upper nibble or results in a digit greater than 9. The same with the upper nibble. How do we know, if an overflow from the lower to the upper nibble has occurred? The MCU sets the H-bit in the status register, the half-carry bit. The following shows the algorithm for the different cases that are possible after adding two nibbles and adding hex 0x6 after that. 1. Add the nibbles. If overflow occurs (C for the upper nibbles, or H for the lower nibbles), add 6 to correct, if not, do step 2.

Loading 250 to a register set representing that number would look like this: LDI R18,'2' LDI R17,'5' LDI R16,'0' The ASCII representation of these characters are written to the registers. Bit manipulations To convert a BCD coded digit to its ASCII representation we need to set bit 4 and 5 to a one. In other words we need to OR the BCD with a constant value of hex 0x30. net OR R1,R2 Back from an ASCII character to a BCD is as easy. The instruction ANDI R1,0x0F isolates the lower four bits (= the lower nibble).