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MC908QT1AMPE Datasheet(Hoja de datos) 67 Page - Freescale Semiconductor, Inc

No. de Pieza. MC908QT1AMPE
Descripción  M68HC08 Microcontrollers
Descarga  200 Pages
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Fabricante  FREESCALE [Freescale Semiconductor, Inc]
Página de inicio  http://www.freescale.com
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 67 page
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CPU Registers
MC68HC908QYA/QTA Family Data Sheet, Rev. 3
Freescale Semiconductor
67
7.3.3 Stack Pointer
The stack pointer is a 16-bit register that contains the address of the next location on the stack. During a
reset, the stack pointer is preset to $00FF. The reset stack pointer (RSP) instruction sets the least
significant byte to $FF and does not affect the most significant byte. The stack pointer decrements as data
is pushed onto the stack and increments as data is pulled from the stack.
In the stack pointer 8-bit offset and 16-bit offset addressing modes, the stack pointer can function as an
index register to access data on the stack. The CPU uses the contents of the stack pointer to determine
the conditional address of the operand.
NOTE
The location of the stack is arbitrary and may be relocated anywhere in
random-access memory (RAM). Moving the SP out of page 0 ($0000 to
$00FF) frees direct address (page 0) space. For correct operation, the
stack pointer must point only to RAM locations.
7.3.4 Program Counter
The program counter is a 16-bit register that contains the address of the next instruction or operand to be
fetched.
Normally, the program counter automatically increments to the next sequential memory location every
time an instruction or operand is fetched. Jump, branch, and interrupt operations load the program
counter with an address other than that of the next sequential location.
During reset, the program counter is loaded with the reset vector address located at $FFFE and $FFFF.
The vector address is the address of the first instruction to be executed after exiting the reset state.
Bit
15
14
13
12
11
10
987654321
Bit
0
Read:
Write:
Reset:
0000000011111111
Figure 7-4. Stack Pointer (SP)
Bit
15
14
13
12
11
10
987654321
Bit
0
Read:
Write:
Reset:
Loaded with vector from $FFFE and $FFFF
Figure 7-5. Program Counter (PC)




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