Commodore VIC-20 - Page 147

If you didn't, then don't worry; learning machine language does take
patience, and cannot be accomplished 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.
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 SFF (hexadecimal),
which equals 255 (decimal). You have probably wondered why only
numbers from zero to two hundred and fifty-five 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 VIC 20 will reply with:
7ILLEGAL QUANTITY
ERROR
READY.
If the limit of one byte is SFF (hex), how is the address parameter
in the absolute instruction "LDA S102E" expressed in memory?
Well, it is 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 6502 requires any address to be specified with its low byte
first, and then the high byte. This means that the instruction "LDA
S102E" is represented in memory by the three consecutive values:
SAD, S2E, S10
We need to know one more instruction before we can write our
first program. That instruction is "BRK." For a full explanation of this
instruction, refer to M.O.S 6502 Programming Manual, You can
think of it as the "END" instruction in machine language.
If we write a program with VICMon and put the BRK instruction at
the end, the program will return to VICMon when it is finished. This
might not happen if there is a mistake in your program, or if the BRK
instruction is never reached (just like an "END" statement in BASIC
may never get executed, and thus if the VIC 20 didn't have a STOP
key, you wouldn't be able to abort your BASIC programs!)
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