MC68HC908GR8 Datasheet, PDF (159/286 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Chapter 17
Random-Access Memory (RAM)
17.1 Introduction
This section describes the 384 bytes of RAM (random-access memory).
17.2 Functional Description
Addresses $0040 through $01BF are RAM locations. The location of the stack RAM is programmable.
The 16-bit stack pointer allows the stack to be anywhere in the 64K byte 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.
MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7
Freescale Semiconductor
159