MC68HC908AT32 Datasheet, PDF (49/378 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 3
Random-Access Memory (RAM)
3.1 Introduction
This section describes the 1024 bytes of random-access memory (RAM).
3.2 Functional Description
Addresses $0050–$044F are RAM locations. The location of the stack RAM is programmable. The 16-bit
stack pointer allows the stack to be anywhere in the 1024-byte memory space.
NOTE
For correct operation, the stack pointer must point only to RAM locations.
Within page zero are 176 bytes of RAM. Because the location of the stack RAM is programmable, all page
zero RAM locations can be used for input/output (I/O) control and user data or code. When the stack
pointer is moved from its reset location at $00FF, direct addressing mode instructions can access all page
zero RAM locations efficiently. Page zero RAM, therefore, provides ideal locations for frequently
accessed global variables.
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 M68HC05, M6805, and M146805 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 could overwrite data
in the RAM during a subroutine or during the interrupt stacking operation.
MC68HC908AT32 Data Sheet, Rev. 3.1
Freescale Semiconductor
49