Datasheet

MC68HC908QY4•MC68HC908QT4•MC68HC908QY2•MC68HC908QT2•MC68HC908QY1•MC68HC908QT1
MOTOROLA Random-Access Memory (RAM) 43
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Data Sheet — MC68HC908QY4
Section 3. Random-Access Memory (RAM)
3.1 Contents
3.2 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
3.3 Functional Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
3.2 Introduction
This section describes the 128 bytes of random-access memory (RAM).
3.3 Functional Description
Addresses $0080–$00FF are RAM locations. The location of the stack
RAM is programmable. The 16-bit stack pointer allows the stack to be
anywhere in the 64-Kbyte memory space.
NOTE: For correct operation, the stack pointer must point only to RAM
locations.
Before processing an interrupt, the central processor unit (CPU) uses
five bytes of the stack to save the contents of the CPU registers.
NOTE: For M6805, M146805, and M68HC05 compatibility, the H register is not
stacked.
During a subroutine call, the CPU uses two bytes of the stack to store
the return address. The stack pointer decrements during pushes and
increments during pulls.
NOTE: Be careful when using nested subroutines. The CPU may overwrite data
in the RAM during a subroutine or during the interrupt stacking
operation.