 Cx51 User's Guide | |
| Home Products Events Support | |
Technical Support
On-Line Manuals
Cx51 User's Guide
Cx51 Introduction Compiling Programs Language Extensions Preprocessor Advanced Programming Customization Files
Startup Code Variable Initialization Code XBANKING.A51 Basic I/O Memory Allocation Optimizer Optimizer Options General Optimizations 8051-Specific Optimizations Global Register Coloring Segment Naming Conventions Data Objects Program Objects Parameters and Local Variables Interfacing C to Assembler Global Variables Function Parameters Passing in Memory Passing in Registers Function Return Values Using the SRC Directive Register Usage Overlaying Segments Small Model Example Compact Model Example Large Model Example Interfacing C programs to PL/M-51 Data Storage Formats Bit Variables 1-Byte Scalars 2-Byte Scalars 4-Byte Scalars Generic and Far Pointers Floating-point Numbers Floating-point Errors Absolute Memory Locations Absolute Access Macros Linker Location Controls The _at_ Keyword Debugging 8051 Derivatives Error Messages Library Reference AppendixGeneric and Far Pointers
Generic pointers have no declared explicit memory type. They may point to any memory area on the 8051. They are stored using three bytes (24 bits). The first byte contains a value that indicates the memory area or memory type. The remaining two bytes contain the address offset with the high-order byte first.
| Address+0 | Address+1 | Address+2 | |
|---|---|---|---|
| Contents | Memory Type | High Byte Offset | Low Byte Offset |
Depending on the compiler version, the memory type byte has the following values:
| Compiler | Memory Type | |||||
|---|---|---|---|---|---|---|
| idata | data | bdata | xdata | pdata | code | |
| C51 Compiler (8051 Devices) | 0x00 | 0x00 | 0x00 | 0x01 | 0xFE | 0xFF |
| CX51 Compiler (NXP 51MX Devices) | 0x7F | 0x7F | 0x7F | 0x00 | 0x00 | 0x80 |
The NXP 80C51MX architecture supports new MCU instructions that operate on a universal pointer. Universal pointers are identical with C51 generic pointers.
Values not specified in the table above access far memory. Generic pointers and far pointers have the same format.
Generic Pointer Example
The following example shows the memory storage of a generic pointer (on the C51 compiler) that references address 0x1234 in the xdata memory area.
| Address+0 | Address+1 | Address+2 | |
|---|---|---|---|
| Contents | 0x01 | 0x12 | 0x34 |
| Contact Site Map Press Privacy | Copyright © 2008 Keil™, An ARM® Company. All rights reserved. |
