ARM assembly language program format

ARM assembly language organizes source files in segments (divided into code segments and data segments). An ARM source program must have at least one code segment, and a large program can contain multiple code segments and data segments.

    Code segments are usually read-only.

    Zero or more data sections containing initial values. Usually readable.

    Zero or more data segments that do not contain initial values. These data segments are initialized to zero and are usually readable.

like:

1. Entrance

AREA Init,CODE,READONLY

 ENTRY

--------------------The ENTRY pseudo-operation identifies the first instruction executed by the program. An ARM program must have at least one ENTRY.

END tells the assembly compiler to end compiling the source file. Every assembly must contain an END

2. Subroutine call

BL Subroutine call name

The BL instruction performs two operations: placing the subroutine return address in the LR register and setting the PC register value to the first instruction address of the target subroutine. Subroutine calls usually use registers R0 to R3 to return results and pass parameters.

There are a lot of subroutine call examples in 2440init.s

;================================================== ==========
ReadNandID
 mov r7,#NFCONF
 ldr r0,[r7,#4] ;NFChipEn();
 bic r0,r0,#2
 str r0,[r7,#4]
 mov r0,# 0x90 ;WrNFCmd(RdIDCMD);
 strb r0,[r7,#8]
 mov r4,#0 ;WrNFAddr(0);
 strb r4,[r7,#0xc]

;================================================== ==========
1 ;while(NFIsBusy());
 ldr r0,[r7,#0x20]
 tst r0,#1
 beq %B1
 ldrb r0,[r7,#0x10] ;id = RdNFDat( )<<8;
 mov r0,r0,lsl #8
 ldrb r1,[r7,#0x10] ;id |= RdNFDat();
 orr r5,r1,r0
 ldr r0,[r7,#4] ;NFChipDs();
 orr r0,r0,#2
 str r0,[r7,#4]
 mov pc,lr 

-----------------------------------Looking at the two examples above, the only difference is that the latter one has mov pc,lr, which indicates that after the program is completed, it will return to the next sentence of the program BL for execution.