Microchip Technology XC8 Standard Compiler (Workstation) SW006021-1 SW006021-1 User Manual
Product codes
SW006021-1
MPLAB
®
XC8 C Compiler User’s Guide
DS52053B-page 54
2012 Microchip Technology Inc.
3.4.7.2
WHAT DO I NEED OTHER THAN INSTRUCTIONS IN AN ASSEMBLY
SOURCE FILE?
SOURCE FILE?
Assembly code typically needs assembler directives as well as the instructions them-
selves. The operation of all the directives are described in the subsections of
Section 6.4.9 “Assembler Directives”. Common directives required are mentioned
below.
selves. The operation of all the directives are described in the subsections of
Section 6.4.9 “Assembler Directives”. Common directives required are mentioned
below.
All assembly code must be placed in a psect so it can be manipulated as a whole by
the linker and placed in memory. See Section 5.15.1 “Program Sections” for general
information on psects; see Section 6.4.9.3 “PSECT” for information on the directive
used to create and specify psects.
the linker and placed in memory. See Section 5.15.1 “Program Sections” for general
information on psects; see Section 6.4.9.3 “PSECT” for information on the directive
used to create and specify psects.
The other commonly used directive is GLOBAL, defined in Section 6.4.9.1 “GLOBAL”
which is used to make symbols accessible across multiple source files.
which is used to make symbols accessible across multiple source files.
3.4.7.3
HOW DO I ACCESS C OBJECTS FROM ASSEMBLY CODE?
Most C objects are accessible from assembly code. There is a mapping between the
symbols used in the C source and those used in the assembly code generated from
this source. Your assembly should access the assembly-equivalent symbols which are
detailed in Section 5.12.3 “Interaction Between Assembly and C Code”.
symbols used in the C source and those used in the assembly code generated from
this source. Your assembly should access the assembly-equivalent symbols which are
detailed in Section 5.12.3 “Interaction Between Assembly and C Code”.
Instruct the assembler that the symbol is defined elsewhere by using the GLOBAL
assembler directive, see Section 6.4.9.1 “GLOBAL”. This is the assembly equivalent
of a C declaration, although no type information is present. This directive is not needed
and should not be used if the symbol is defined in the same module as your assembly
code.
assembler directive, see Section 6.4.9.1 “GLOBAL”. This is the assembly equivalent
of a C declaration, although no type information is present. This directive is not needed
and should not be used if the symbol is defined in the same module as your assembly
code.
Any C variable accessed from assembly code will be treated as if it were qualified vol-
atile
atile
, see Section 5.4.7.2 “Volatile Type Qualifier”. Specifically specifying the
volatile
qualifier in C code is preferred as it makes it clear that external code may
access the object.
3.4.7.4
HOW CAN I ACCESS SFRS FROM WITHIN ASSEMBLY CODE?
The safest way to gain access to SFRs in assembly code is to have symbols defined
in your assembly code that equate to the corresponding SFR address. Header files are
provided with the compiler so that you do not need to define these yourselves, and they
are detailed in Section 5.12.3.2 “Accessing Registers from Assembly Code”.
in your assembly code that equate to the corresponding SFR address. Header files are
provided with the compiler so that you do not need to define these yourselves, and they
are detailed in Section 5.12.3.2 “Accessing Registers from Assembly Code”.
There is no guarantee that you will be able to access symbols generated by the com-
pilation of C code, even code that accesses the SFR you require.
pilation of C code, even code that accesses the SFR you require.
3.4.7.5
WHAT THINGS MUST I MANAGE WHEN WRITING ASSEMBLY CODE?
If you are hand-writing assembly code there are several things that you must take con-
trol of.
trol of.
• Whenever accessing a RAM variable, you must ensure that the bank of the vari-
able is selected before you read or write the location. This is done by one or more
assembly instructions. The exact code is based on the device you are using and
the location of the variable. Bank selection is not be required if the object is in
common memory, (which is called the access bank on PIC18 devices) or if you
are using an instruction that takes a full address (such as the MOVFF instruction on
PIC18 devices). Check your device data sheet to see the memory architecture of
your device, and the instructions and registers which control bank selection. Fail-
ure to select the correct bank will lead to code failure.
The BANKSEL pseudo instruction can be used to simplify this process, see
Section 6.4.1.2 “Bank and Page Selection”.
assembly instructions. The exact code is based on the device you are using and
the location of the variable. Bank selection is not be required if the object is in
common memory, (which is called the access bank on PIC18 devices) or if you
are using an instruction that takes a full address (such as the MOVFF instruction on
PIC18 devices). Check your device data sheet to see the memory architecture of
your device, and the instructions and registers which control bank selection. Fail-
ure to select the correct bank will lead to code failure.
The BANKSEL pseudo instruction can be used to simplify this process, see
Section 6.4.1.2 “Bank and Page Selection”.