Commodore Plus 4 - Page 252

240 Machine Language on the Commodore Plus/4
Bit
Label Name
Usage
5
4 B
D
Not Used
Break
Decimal
Interrupt
Zero
Carry
This bit is set to 1 only when the last instruc
tion was a BRK, and is cleared to 0
otherwise.
This bit is set to 1 when the microprocessor is
operating in decimal mode and is cleared to
0 otherwise.
When this bit is set to 1, maskable interrupts
are not performed. When it is cleared to 0
interrupts are restored.
This bit is set to 1 when the last result was zero
and cleared to 0 when the last result was
nonzero.
The carry is set to 1 if the last addition
resulted in a carry or the last subtraction did
not require a borrow. Otherwise it is cleared
toO.
The Accumulator The accumulator (.A) is an 8-bit general purpose register. It
is generally used as the main communication register between segments of code.
TheXRegister The X register (.X) is an 8-bit index register. It may be used as an
index (see sections on indexed and indexed indirect addressing) or as temporary
storage for intermediate results.
The YRegister The Y register (. Y) is an 8-bit index register. It may be used as an
index (see sections on indexed and indirect indexed addressing) or as temporary
storage for intermediate results.
The Stack Pointer The stack pointer (SP) is an 8-bit register that contains the
current pointer to the stack on page 1 (see the next section on the stack).
The Stack
The 6502 maintains a 256-byte stack that is always located on page 1 (memory
locations $0100 through S01FF). The stack pointer register (SP) maintains the
current position within the stack. It is initially set by the operating system at $FF
to point to the first position ($01FF). When an item is pushed onto the stack, it is