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发表于 2006-4-30 21:41:45
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Inline Assembly:
Besides writing assembly functions in assembly files, inline assembly allows you to write assembly code within your C file. (Of course, you may use assembly source files as part of your project as well.) The syntax for inline assembly is:
asm("<string>");
Multiple assembly statements can be separated by the newline character
. String concatenations can be used to specify multiple statements without using additional asm keywords. To access a C variable inside an assembly statement, use the %<name> format:
register unsigned char uc;
asm("mov %uc,R0
"
"sleep
");
Any C variable can be referenced this way (excluding C goto labels). If you use an assembly instruction that requires a CPU register, then you must use the register storage class to force a local variable to be allocated in a CPU register.
In general, using inline assembly to reference local registers is limited in power: it is possible that no CPU registers are available if you have declared too many register variables in that function. In that case, you would get an error from the assembler. There is also no way to control allocation of register variables, so your inline instruction may fail. For example, using the ldi instruction requires the register to be one of the 16 upper registers, but there is no way to request that using inline assembly. There is also no way to reference the upper half of an integer register.
Inline assembly may be used inside or outside of a C function. The compiler indents each line of the inline assembly for readability. Unlike the AVR assembler, the ImageCraft assembler allows labels to be placed anywhere (not just at the first character of the lines in your file) so you may create assembly labels in your inline assembly code. You may get a warning on asm statements that are outside of a function. You may ignore these warnings.
Assembly Interface and Calling Conventions:
External Names
External C names are prefixed with an underscore, e.g. the function main is _main if referenced in an assembly module. Names are significant to 32 characters. To make an assembly object global, use two colons after the name. For example,
_foo::
.word 1
(In the C file)
extern int foo;
Argument and Return Registers
The first argument is passed in registers R16/R17 if it is an integer and R16/R17/R18/R19 if it is a long or floating point. The second argument is passed in R18/R19 if available. All other remaining arguments are passed on the software stack.
Integer arguments smaller than ints (i.e. char) are not extended to in IF the function prototype is available. If registers are used to pass byte arguments, it will use both registers but the higher register is undefined. For example, if the first argument is a byte, both R16/R17 will be used with R17 being undefined. Byte arguments passed on the software stack take up only a single byte. In the future release, byte arguments passed in registers will be packed tightly.
If R16/R17 is used to pass the first argument and the second argument is a long or float, the lower half of the second argument is passed in R18/R19 and the upper half is passed on the software stack.
Integer values are returned in R16/R17 and longs and floats are returned in R16/R17/R18/R19. Byte values are returned in R16 with R17 undefined.
Preserved Registers
An assembly function must save and restore the following registers:
R28/R29 or Y, this is the frame pointer.
R10/R11/R12/R13/R14/R15/R20/R21/R22/R23
These registers are called preserved registers since their contents are unchanged by a function call. You can ask the compiler not to use the registers R20, R21, R22, and R23, then you do not need to save and restore these 4 registers.
Volatile Registers
The other registers:
R0/R1/R2/R3/R4/R5/R6/R7/R8/R9/R24/R25/R26/R27/R30/R31
SREG
can be used in a function without being saved or restored. These registers are called volatile registers since their contents may be changed by a function call.
Interrupt Handlers
Note that unlike a normal function, an interrupt handler must save and restore all registers that it uses. This is done automatically if you use the compiler capability to declare a C function as an interrupt handler. If you write a handler in assembly and if it calls normal C functions, then the assembly handler must save and restore the Volatile Registers since normal C functions do not preserve them. Since an interrupt handler operates asynchronous to the normal program operation, the interrupt handler or the functions it calls must not modify any machine registers. The exception is when you ask the compiler not to use the registers R20, R21, R22, and R23, then the interrupt handlers may use these 4 registers directly. |
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