can distinguish between the immediate mode, and the absolute mode of the LDA instruction, because they have slightly different tokens. LDA (immediate) has $A9 as its token, and LDA (absolute), has $AD as its token.
The mnemonic representing an instruction usually implies what it does. For instance, if we consider another instruction, LDX, what do you think this does?
If you said "load the X register with...", go to the top of the class. If you didn't, then don't worry, learning machine language does take patience, and cannot be learned in a day.
The various internal registers can be thought of as special memory locations, because they too can hold one byte of information. It is not necessary for us to explain the binary numbering system (base 2) since it follows the same rules as outlined for hexadecimal and decimal outlined previously, but one "bit" is one binary digit and eight bits make up one byte! This means that the maximum number that can be contained in a byte is the largest number that an eight digit binary number can be. This number is 11111111 (binary), which equals $FF (hexadecimal), which equals 255 (decimal). You have probably wondered why only numbers from zero to 255 could be put into a memory location. If you try POKE 7680,260 (which is a BASIC statement that "says": "Put the number two hundred and sixty, into memory location seven thousand, six hundred and eighty", the BASIC interpreter knows that only numbers 0 - 255 can be put in a memory location, and your Commodore 64 will reply with:
?ILLEGAL QUANTITY ERROR READY.
If the limit of one byte is $FF (hex), how is the address parameter in the absolute instruction "LDA $102E" expressed in memory? It's expressed in two bytes (it won't fit into one, of course). The lower (rightmost) two digits of the hexadecimal address form the "low byte" of the address, and the upper (leftmost) two digits form the "high byte."
The 6510 requires any address to be specified with its low byte first, and then the high byte. This means that the instruction "LDA $102E" is represented in memory by the three consecutive values:
$AD, $2E, $10
Now all you need to know is one more instruction and then you can write your first program. That instruction is BRK. For a full explanation of this instruction, refer to M.O.S. 6502 Programming Manual. But right now, you can think of it as the END instruction in machine language.
|This page has been created by Sami Rautiainen.|
|Read the small print.||Last updated July 10, 2002.|