TQ2440 startup code analysis (Part 3)

The entry address must be specified in the initialization program, because after the processor is reset (when emulating, loading the image), the PC must find the entry to start executing the code. When various exceptions or interrupts occur, the entry of each exception must also be found to start executing the code.

From here on is the real code entry!

 

The AREA directive is used to define a code segment or a data segment. The ARM assembler is designed in a segmented manner. An
ARM source program requires at least one code segment. A large program can contain as many code segments and data segments as desired.

CODE   is the definition code segment. The default attribute is READONLY;

READONLY  specifies that this section is read-only. The default attribute of the code section is READONLY.

;================================================== =========

;After the board is powered on and reset, the program starts to execute from 0x0. After the hardware is powered on and reset, the program starts to execute from here and jumps to the ResetHandler.

;DCD is used to allocate a single word memory chip and initialize it with the following pseudo instruction. The allocated bytes are determined by the number of expr

 

      AREA    Init,CODE,READONLY; declare a code segment named Init, ;Init initialization program

      ENTRY   program entry point, (for debugging)

      ;ENTRY is just a common entry point, and it can be defined in multiple places in the program.

The only entry point for an image file also requires setting relevant options in the linker.

      EXPORT __ENTRY

__ENTRY

ResetEntry  ;Entry after reset

      ;1)The code, which converts to Big-endian, should be in little endian code.

      ;2)The following little endian code will be compiled in Big-Endian mode.

      ;  The code byte order should be changed as the memory bus width.

      ;3)The pseudo instruction,DCD can not be used here because the linker generates error.

      ASSERT :DEF:ENDIAN_CHANGE  

;ASSERT assertion error pseudo instruction, here indicates whether ENDIAN_CHANGE has been defined

      [ ENDIAN_CHANGE  ; If ENDIAN_CHANGE is defined

             ASSERT  :DEF:ENTRY_BUS_WIDTH  ; This indicates whether ENTRY_BUS_WIDTH has been defined

; If ENTRY_BUS_WIDTH is not defined, an error will be reported

             [ ENTRY_BUS_WIDTH=32   ;if  ENTRY_BUS_WIDTH=32

                    b     ChangeBigEndian     ; Jump to ChangeBigEndian (see the red code below), execute DCD 0xea000007 to change the big-endian and small-endian data modes

             ]  ;end  if

 

             [ ENTRY_BUS_WIDTH=16  ;if ENTRY_BUS_WIDTH=16

                    andeq      r14,r7,r0,lsl #20   ;When the Z bit of the flag status register CPSR = 1, r14 = r7 + r0 logically shifts left by 20 bits, and executes DCD 0x0007ea00 to change the big-endian mode

             ] ;end if

 

             [ ENTRY_BUS_WIDTH=8  ;if  ENTRY_BUS_WIDTH=8

                    streq       r0,[r0,-r10,ror #1] ; When the Z bit of the flag status register CPSR = 1..., execute DCD 0x070000ea to change the big-endian mode

             ]

             |  ;ELSE if ENDIAN_CHANGE is not defined

             b     ResetHandler; reset abnormality, the development board enters 0x00 when powered on or reset

      ]

The interrupt vector table is generally located at the beginning of the startup code. It is between the user program and the startup code and between the startup code

It is a link between the various parts of the system. It consists of jump functions one by one, and it is like an ordinary jump function, except that

However, the process of scattering involves hardware mechanisms. When an exception occurs in the system, the ARM processor will force

The PC pointer points to the address of the corresponding exception jump function in the interrupt vector table, and then the program will jump to the corresponding

The service program is interrupted to execute.

b     HandlerUndef ;handler for Undefined mode Undefined exception, 0x04 when encountering an unrecognizable instruction

b     HandlerSWI   ;handler for SWI interrupt, soft interrupt exception 0x08 

b     HandlerPabort ;handler for PAbort instruction prefetch error enters 0x0c  processing termination program access termination mode

b     HandlerDabort       ;handler for DAbort enters 0x10  processing data access termination mode when data access cannot be completed

b     .             ;reserved, reserved 0x14  "." represents the address of the instruction, which means an infinite loop

b     HandlerIRQ    ;handler for IRQ interrupt When an IRQ interrupt occurs, enter 0x18

b     HandlerFIQ    ;handler for FIQ interrupt When a FIQ interrupt occurs, enter 0x1c

 

;@0x20 "@" Current value of the storage location counter

b     EnterPWDN   ; Must be @0x20.

158 ;================================================ ===============

159; The following is a program to change the big and small endianness. Here we use the method of directly defining the machine code. As for why we do this

160; You have to ask Samsung. Anyway, this code will not be executed in our program, so don't worry about it.

161 ;================================================ ===============

ChangeBigEndian  ; Change the big-endian and small-endian data modes

;@0x24

      [ENTRY_BUS_WIDTH=32

DCD is used to allocate a word memory unit and initialize it with expr in the pseudo instruction. The memory allocated by the DCD pseudo instruction
needs to be word aligned and can generally be used to define data tables or other constants. & is synonymous with DCD.

             DCD       0xee110f10     ;0xee110f10 => mrc p15,0,r0,c1,c0,0

             DCD       0xe3800080    ;0xe3800080 => orr r0,r0,#0x80;  //Big-endian

             DCD       0xee010f10     ;0xee010f10 => mcr p15,0,r0,c1,c0,0

      ]

      [ENTRY_BUS_WIDTH=16

             DCD 0x0f10ee11

             DCD 0x0080e380

             DCD 0x0f10ee01

      ]

      [ENTRY_BUS_WIDTH=8

             DCD 0x100f11ee

             DCD 0x800080e3

             DCD 0x100f01ee

      ]

      DCD 0xffffffff  ;swinv 0xffffff is similar with NOP and run well in both endian mode.

      DCD 0xffffffff

      DCD 0xffffffff

      DCD 0xffffffff

      DCD 0xffffffff

      b ResetHandler  ; Reset handler

 

; Enter power-down mode function

; 1. SDRAM must be in self-refresh mode.

; 2. All interrupts must be masked for SDRAM/DRAM self-refresh.

; 3. LCD off for SDRAM/DRAM self-refresh.

; 4. The I-cache may need to be enabled.

; 5. The location of the following code may have not to be changed.

 

; As mentioned above, the HANDLER macro is used here to establish the connection between Hander*** and Handle***

HandlerFIQ    HANDLER HandleFIQ     ;HandlerIRQ Line number    HANDLER Macro name  HandleIRQ Imported parameters

HandlerIRQ   HANDLER HandleIRQ

HandlerUndef HANDLER HandleUndef

HandlerSWI   HANDLER HandleSWI

HandlerDabort       HANDLER HandleDabort

HandlerPabort HANDLER HandlePabort