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Compiler Runtime Environment
Without the global pointer, loading data from a static memory area takes two
instructions – one to load the most significant bits of the 32-bit constant address
computed by the compiler/linker and one to do the data load.
To utilize gp -relative addressing, the compiler and assembler must group all of the
“small” variables and constants into one of the following sections:
? .lit4.
? .sdata.
? .sdata.*
? .gnu.linkonce.s.*
? lit8
? sbss
? sbss.*
? .gnu.linkonce.sb.*
The linker must then group all of the above input sections together. The run-time startup
code must initialize the gp register to point to the “middle” of this output section. To
enable the startup code to initialize the gp register, the linker script must initialize a
variable which is 32 KB from the start of the output section containing the “small”
variables and constants. This variable is named _gp (to match core linker scripts).
Besides being initialized in the standard GPR set, the global pointer must also be
initialized in the register shadow set.
FIGURE 5-5:
GLOBAL POINTER LOCATION
5.7.2.4
CALL “ON RESET” PROCEDURE
A procedure is called after initializing a minimum ‘C’ context. This procedure allows
users to perform actions almost immediately on reset of the device. An empty weak
version of this procedure ( _on_reset ) is provided with the startup code. Special
considerations needs to be taken by the user if this procedure is written in 'C'. Most
importantly, statically allocated variables are not initialized (with either the specified
initializer or a zero as required for uninitialized variables).
5.7.2.5
CLEAR UNINITIALIZED DATA SECTIONS
There are two uninitialized data sections— .sbss and .bss . The .sbss section is a
data segment containing uninitialized variables less than or equal to n bytes where n
is determined by the - G n command line option. The .bss section is a data segment
containing uninitialized variables not included in .sbss .
? 2007 Microchip Technology Inc.
DS51686A-page 63
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相关PDF资料
SW300003-EVAL
LIBRARY SOFT MODEM-EVAL ONLY
SW300010-EVAL
SPEECH RECOG LIBRARY-EVAL ONLY
SW300040-EVAL
LIBRARY NOISE SUPPR-EVAL ONLY
SW300060-EVAL
LIBRARY ACOUSTIC ECHO-EVAL ONLY
SW300070-EVAL
SOFTWARE LIBR ENC/DEC EVAL ONLY
SW500012
HI-TECH C PRO FOR PIC32
SY87701LHI-EVAL
BOARD EVAL SY87701 EXPERIMENT
SY87729LHI-EVAL
BOARD EVAL N SY87729 EXPERIMENT
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