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Chapter 6. Advanced Programming Techniques |
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Accessing Absolute Memory Locations
The C programming language does not support a method of explicitly specifying the memory location of a static or global variable. There are three ways to reference explicit memory location. You can use the:
Absolute memory access macros
Linker location controls
The _at_ keyword
Each of these three methods is described below.
Absolute Memory Access Macros
First, you may use the absolute memory access macros provided as part of the Cx51 library. Use the following macros to directly access the memory areas of the 8051.
CBYTE |
FCVAR |
CWORD |
DBYTE |
FVAR |
DWORD |
FARRAY |
PBYTE |
PWORD |
FCARRAY |
XBYTE |
XWORD |
Refer to “Absolute Memory Access Macros” on page 212 for definitions of these 6 macros.
Keil Software — Cx51 Compiler User’s Guide |
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Linker Location Controls
The second method of referencing explicit memory location is to declare the variables in a stand-alone C module, and use the location directives of the BL51 Linker/Locator to specify an absolute memory address.
In the following example, assume that we have a structure called alarm_control that we want to reside at address 2000h in xdata. We start by entering a source file named ALMCTRL.C that contains only the declaration for this structure.
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struct alarm_st {
unsigned int alarm_number; unsigned char enable flag; unsigned int time_delay; unsigned char status;
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xdata struct alarm_st alarm_control; |
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The Cx51 compiler generates an object file for ALMCTRL.C and includes a |
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segment for variables in the xdata memory area. Because it is the only variable |
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declared in this module, alarm_control is the only variable in that segment. |
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The name of the segment is ?XD?ALMCTRL. The Lx51 Linker/Locator allows |
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you to specify the base address of any segment by using the location directives. |
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For BL51 you must use the XDATA directive, since the alarm_control |
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variable was declared to reside in xdata: |
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BL51 ... |
almctrl.obj XDATA(?XD?ALMCTRL(2000h)) … |
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For LX51 the SEGMENTS directive is used to locate the segment in xdata space:
LX51 ... almctrl.obj SEGMENTS(?XD?ALMCTRL(X:0x2000)) …
This instructs the linker to locate the segment named ?XD?ALMCTRL at address 2000h in the xdata memory area.
In the same way you may also locate segments in the other memory areas like code, xdata, pdata, idata, and data. Refer to the A51 Macro Assembler User’s Guide for more information about the Linker/Locator.
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The _at_ Keyword
The third method of accessing absolute memory locations is to use the _at_ keyword when you declare variables in your C source files. The following example demonstrates how to locate several different variable types using the _at_ keyword.
struct link |
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struct link idata *next; |
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*test; |
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struct link list idata _at_ 0x40; |
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char xdata text[256] |
_at_ 0xE000; |
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int xdata i1 |
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_at_ 0x8000; |
void main ( void ) { link.next = (void *) 0; i1 = 0x1234; text [0] = 'a';
}
/* list at idata 0x40 */ /* array at xdata 0xE000 */ /* int at xdata 0x8000 */
Refer to “Absolute Variable Location” on page 104 for more information about the _at_ keyword.
NOTE
If you use the _at_ keyword to declare a variable that accesses an XDATA peripheral, you may require the volatile keyword to ensure that the C compiler does not optimize out necessary memory accesses.
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