Analysis of technical difficulties in starting program in embedded applications using ARM7

Analysis of technical difficulties

⑴.Use of MMU

MMU is the abbreviation of memory management unit, which is a device used to manage the virtual memory system. MMU is usually part of the CPU and has a small amount of storage space to store the matching table from virtual address to physical address. This table is called TLB (Translation Lookaside Buffer). All data requests are sent to the MMU, which determines whether the data is in RAM or in a large-capacity storage device. If the data is not in the storage space, the MMU will generate a page fault interrupt.

The two main functions of the MMU are:

Convert a virtual address to a physical address.

Control memory access permissions. When the MMU is turned off, the virtual address is directly output to the physical address bus.

In practice, using MMU solves the following problems:

① When using DRAM as a large-capacity memory, if the rows and columns of DRAM are non-square, the physical address of the DRAM will be discontinuous, which will cause great inconvenience to program writing and debugging. Proper configuration of the MMU can convert it into a continuous virtual address space.

②The interrupt vector table of the ARM core is required to be placed at address 0. When the ROM is at address 0, the interrupt service program cannot be debugged, so it is necessary to map the readable and writable memory space to address 0 during the debugging phase.

③ Certain address segments of the system are not allowed to be accessed, otherwise unpredictable consequences will occur. In order to avoid such errors, these address segments can be set to user-inaccessible types through the settings of the MMU matching table.

The matching table generated by the boot program contains information such as address mapping, storage page size (1M, 64K, or 4K), and whether access is allowed.

For example: the physical address range of the 16M DRAM on the target board is 0xc000, 0000~0xc07f, ffff; 0xc100, 0000~0xc17f, ffff; the virtual address range of the 16M ROM is: 0x0000, 0000~0x00ff, ffff. The matching table is configured as follows:

It can be seen that the left side is a continuous virtual address space, the right side is a discontinuous physical address space, and DRAM is mapped to the address interval 0. The MMU obtains the corresponding physical address according to the conversion logic based on the virtual address and page table location information, and outputs it to the address bus.

It should be noted that after enabling the MMU, the program continues to run, but for the programmer, the pointer of the program counter has changed to point to the virtual address corresponding to the ROM.

⑵Distribution loading analysis of target files

First, create a text file, called a distribution load description file, which specifies the loading range and execution range for each part of the application.

Here are some examples:

FLASH 0x01000000 0x011fffff; 2M FLASH

{

FLASH 0x01000000

{

boot.o(BOOT,+FIRst)

* (+RO)

}

DRAM 0x00000000

{

vector.0(VECTOR, +First)

int_handler.o (+RO)

* (+RW, +ZI)

}

}

Add “-scov description-file –scf” or “-scatter description-file” to the command line of the ARM linker. After compilation and linking, a distribution loading file will be generated.

The linker also generates a set of symbols that give the length, load address, and execution address of each interval named in the distribution description file. Since neither the linker nor the C library has the function of copying code from its load interval to the execution interval, or creating a zero-initialized area, it is up to the application programmer to use the information generated by this set of symbols to complete this work. This must be done before calling the C program, as shown below:

LDR r0, = |Load$$DRAM$$Base|

LDR r1, = |Image$$DRAM$$Base|

CMP r0, r1 ; Check if the load address and execution address are the same

BEQ do_zi_init ; If the interval is the same, the interval will not be copied and the zero data area will be initialized

MOV r2, r1 ; different, copy the load area to the execution area

LDR r4, = |Image$$DRAM$$length|

ADD r2, r2, r4

BL copy

do_zi_init

LDR r1, = |Image$$DRAM$$ZI$$Base|

MOV r2, r1

LDR r4, = |Image$$DRAM$$ZI$$length|

ADD r2, r2, r4

MOV r3, #0

BL zi_init ; Call zero initialization subroutine

Conclusion:

The startup program introduced in this article has been run and tested on systems developed with Cirrus Logic's EP7211 and Ateml's AT91M40400. In the future, we can add serial communication modules and FLASH operation modules to develop system monitoring programs, thereby realizing online upgrades of application programs.