MC68HC908GR4MP datasheet(159/286 Pages) FREESCALE

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MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

Freescale Semiconductor

159

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.

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