Commodore VIC-20 - Page 150

backwards in memory, f ,e., the first position in the stack is at $01FF,
and the last is at SOT 00. Another register in the 6502 microproces
sor that hasn't been mentioned yet is called the "stack pointer," and
it always points at the next available location En the stack. When
something is pushed onto the stack, it is placed where the stack
pointer points toT and the stack pointer is moved down to the next
position (decremented). When something is pulled oft the stack, the
stack pointer is incremented, and the byte pointed to by the stack
pointer (at S0100 offset by the contents of the stack pointer) is
placed into the specified register.
Up to this point, we have covered immediate, zero page, and
absolute mode instructions. We have also covered (but have not
stated) the "implied11 mode, which means that the instruction itself
tells what registers/flags/memory the instruction is referring to. The
examples we have seen are PHA, PLAT PHP, and PLP, which refer
to stack processing and the accumulator and status registers. i
The X register will be referred to as X from now on, and similarly
with A - accumulator, Y - Y index register, S - stack pointer, and
P - processor status).
INDEXING
Indexing plays an extremely important part in the running oi the
6502 microprocessor. It can be defined as "creating an actual
address from a base address plus the contents of either the X or Y
index registers."
For example, if X contains $05, and the microprocessor executes
an LDA instruction in the "absolute X indexed mode11 with base
address, e.g., S9Q0Q, then the actual location that is loaded into the
A register is S9000 + S05 = S9005. The mnemonic format of an
absolute indexed instruction is the same as an absolute [nstruction
except a ",X" or ",Y" denoting the index is added to the address,
e.g.:
LDA S9000,X
INDIRECT INDEXED ADDRESSING
This mode allows the program to choose a memory location from
256 adjacent locations. The address of the lowest location is stored
in zero page, and the value in the Y register is added to that address
to choose the final address.
For example, we will place a S45 in location $01, and a $1E in
location $02, We will use the instruction to load the accumulator in
the indirect indexed modef specifying zero page address S01 as the
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