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C251 User's Guide
C251 Introduction Compiling Programs Language Extensions Preprocessor 
Advanced Programming Customization Files 
Startup Code Variable Initialization Code Basic I/O Memory Allocation Optimizer General Optimizations 251-Specific Optimizations Options for Code Generation Global Register Coloring Segment Naming Conventions Segment Types Initialization Segments Function Symbols Parameter Passing Symbols Hardware Stack Calculating Stack Size Interfacing C Programs to Assembler Global Variables Function Parameters Passing in Memory Passing in Registers Function Return Values Using the SRC Directive Register Usage Overlaying Segments TINY Model XTINY Model SMALL Model XSMALL Model LARGE Model Data Storage Formats Bit Variables 1-Byte Scalars 2-Byte Scalars 4-Byte Scalars Floating-point Numbers Floating-point Errors Double Numbers Double Errors Absolute Memory Locations Absolute Access Macros Linker Location Controls The _at_ Keyword Debugging Error Messages Library Reference AppendixLinker Location Controls
Another method of referencing explicit memory locations is to declare the variables in a stand-alone C module and use the L251 linker location directives to specify at an absolute memory address.
In the following example, the alarm_control structure is defined containing the variables to locate.
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struct alarm_st {
unsigned int alarm_number;
unsigned char enable flag;
unsigned int time_delay;
unsigned char status;
};
struct alarm_st near alarm_ctrl;
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The C251 Compiler generates an object file for the above source (ALMCTRL.C) and includes a segment for variables in the near memory type. Since alarm_ctrl is the only variable declared in this module, it is the only variable in the near segment. The name of the segment is ?ED?ALMCTRL.
The L251 linker can locate a segment at any address by using the location control directive SEGMENTS. The L251 SEGMENTS directive must be used as follows:
L251 ... almctrl.obj SEGMENTS(?ED?ALMCTRL(0x2000h)) ...
This instructs the linker to locate the segment named ?ED?ALMCTRL at address 02000h in the linear 16MB address space.
You may also locate segments in the other memory areas like code, xdata, pdata, idata, and data. Refer to the L251 Linker User's Guide for more information.
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