MC68HC908GZ8 Datasheet, PDF (217/344 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Data Sheet — MC68HC908GZ8
Section 18. Random-Access Memory (RAM)
18.1 Introduction
This section describes the 1024 bytes of RAM (random-access memory).
18.2 Functional Description
Addresses $0040 through $043F 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.
Within page zero are 192 bytes of RAM. Because the location of the stack RAM is
programmable, all page zero RAM locations can be used for I/O control and user
data or code. When the stack pointer is moved from its reset location at $00FF out
of page zero, direct addressing mode instructions can efficiently access all page
zero RAM locations. Page zero RAM, therefore, provides ideal locations for
frequently accessed global variables.
Before processing an interrupt, the CPU uses five bytes of the stack to save the
contents of the CPU registers.
NOTE: For M6805 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.
MC68HC908GZ8
MOTOROLA
Random-Access Memory (RAM)
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Data Sheet
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