Xilinx EDK 8.2i manual Memory Model

Memory Model

Figure 3-2: Stack Frame

Calling Convention

R

The caller function passes parameters to the callee function using either the registers (R5 through R10) or on its own stack frame. The callee uses the caller’s stack area to store the parameters passed to the callee.

Refer to Figure 3-2. The parameters for Func 2 are stored either in the registers R5 through R10 or on the stack frame allocated for Func 1.

Memory Model

The memory model for MicroBlaze classifies the data into four different parts:

Small data area

Global initialized variables which are small in size are stored in this area. The threshold for deciding the size of the variable to be stored in the small data area is set to 8 bytes in the MicroBlaze C compiler (mb-gcc), but this can be changed by giving a command line option to the compiler. Details about this option are discussed in the GNU Compiler Tools chapter. 64K bytes of memory is allocated for the small data areas. The small data area is accessed using the read-write small data area anchor (R13) and a 16-bit offset. Allocating small variables to this area reduces the requirement of adding Imm instructions to the code for accessing global variables. Any variable in the small data area can also be accessed using an absolute address.

Data area

Comparatively large initialized variables are allocated to the data area, which can either be accessed using the read-write SDA anchor R13 or using the absolute address, depending on the command line option given to the compiler.

Common un-initialized area

Un-initialized global variables are allocated in the common area and can be accessed either using the absolute address or using the read-write small data area anchor R13.

Literals or constants

Constants are placed into the read-only small data area and are accessed using the read- only small data area anchor R2.

The compiler generates appropriate global pointers to act as base pointers. The actual values of the SDA anchors are decided by the linker, in the final linking stages. For more information on the various sections of the memory please refer to the Address Management chapter. The compiler generates appropriate sections, depending on the command line options. Please refer to the GNU Compiler Tools chapter for more information about these options.

MicroBlaze Processor Reference Guide	www.xilinx.com	69
UG081 (v6.0) June 1, 2006	1-800-255-7778