Commodore VIC-20 - Page 149

The G should turn into an "A" if you have done everything
correctly. You have now written your first machine language
program! Its purpose is to store one character, the letter A, in the
first byte of screen memory.
ADDRESSING MODES
ZERO PAGE
As shown earlier, absolute addresses are expressed in terms of
a high order and a low order byte. The high order byte is often
referred to as the page of memory. For example, the address S1637
is in page S16 (22), and S0277 is in page S02 (2). There is, however,
a special mode of addressing known as "zero page" addressing
and it is. as the name implies, associated with the addressing of
memory locations in page zero. These addresses have a high order
byte of zero. The zero page mode of addressing only expects one
byte to describe the address, rather than two when using an
absolute address, which saves speed and time. This mode tells the
microprocessor to assume that the high order address is zero.
Therefore zero page addressing can reference memory locations
whose addresses are between SOOOO, and SOOFF.
THE STACK
The 6502 microprocessor (like almost all others) has what is
known as a "stack." This is used both by the programmer and the
microprocessor to temporarily remember things, and to remember
the order of events. The GOSUB statement in BASIC, which allows
the programmer to call a "subroutine," must remember where it is
being called from. When the RETURN statement is executed in the
subroutine, the BASIC interpreter "knows" where to go back in
order to continue executing. When a GOSUB statement is
encountered in a program by the BASIC interpreter, the BASIC
interpreter "pushes" its current position onto the stack before going
to do the subroutine, and when a RETURN is executed, the
interpreter "pulls" from the stack the information that tells it where it
was before the subroutine call was made, so that it may continue as
if nothing had happened. The interpreter uses instructions like PHA
which will push the contents ol the accumulator onto the stack, and
PLA (the inverse) which will pull a value off the stack into the
accumulator. The status register can also be pushed and pulled
with the PHP, and PLP respectively.
The stack is 256 bytes long, and is located in page one of
memory. It is therefore from S0100 to S01FF. It is organized
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