Commodore VIC-20 - Page 152

page zero is the actual address because the index has already
been used in the indirection.
Let us fill location $05 with $45, and location S06 with S1E. If the
instruction to load the accumulator in the indexed indirect mode is
executed and the specified zero page address is S01, then the
actual address will be comprised of:
low order = contents of (S01 +X)
high order - contents of (S02 + X)
X register = $04
Thus the actual address will be in = S01 * X - SOS
Therefore, the actual address will be the indirect address
contained In S05 and $06 which is $1E45
This is like sending a mailing to a specific list ot addresses. We
will store a list in zero page, and send the "letter" only to those in the
list. Suppose the list of addresses starts at $00. Here is a program
to send a "letter" to one of the addresses:
LDA #500 —load low order actual base address
STA S06 —set the low byte of the indirect address
LDA #516 —load high order indirect address
STA $07 —set the high byte of the indirect address
LDX #306 —set the indirect index (X)
LDA ($00,X) —load indirectly indexed by X,
BRANCHES AND TESTING
Another very important principle in machine language is the
ability to lest, and detect certain conditions, in a similar fashion to
the "IF. . THEN" structure En VIC BASIC.
The various "flags" in the status register are affected by different
instructions in different ways. For example, there is a flag 1hat is set
when an instruction has caused a zero result, and is reset when a
result is not zero.
LDA #$00
This instruction will cause [Hthe zero result flag" to be set,
because the instruction has resulted in the accumulator containing
a zero.
There is a set of instructions that will, given a particular condition,
"branch11 to another part of the program. An example of a branch
instruction is "'BEQ11, which means "branch if result equal to zero."
The branch instructions will "branch" if the condition is true, and if
not, the program will continue onto the next instruction, as if nothing
had occurred. The branch instructions branch not by the result of
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