二次bootloader关于boot28.asm应用的注意事项，28035的

做BL的时候，主函数被改成了BLmian（）

boot28.asm中 LCR _SBLmain，如此带返回调用主函数，这句话运行结束后，SBLmain函数的返回值是个APP的入口地址放在ACC中，通过 LCR _exit 调用，最终将ACC中的主函数入口地址加载到PC中

因此， _exit函数的内容要加到boot28.asm中，不然引用的_exit是库中的，没有上面这个过程，就无法完成跳转

添加到boot28.asm中的代码如下：

_exit:

;-----------------------------------------------
;   Insure that the stack is deallocated
;-----------------------------------------------

    MOV SP,#__stack

;-----------------------------------------------
; Clear the bottom of the stack.  This will endup
; in RPC when we are finished
;-----------------------------------------------

    MOV  *SP++,#0
    MOV  *SP++,#0

;-----------------------------------------------
; Load RPC with the entry point as determined
; by the boot mode.  This address will be returned
; in the ACC register.
;-----------------------------------------------

    PUSH ACC
    POP  RPC

;-----------------------------------------------
; Put registers back in their reset state.
;
; Clear all the XARn, ACC, XT, and P and DP
; registers
;
; NOTE: Leave the device in C28x operating mode
;       (OBJMODE = 1, AMODE = 0)
;-----------------------------------------------
    ZAPA
    MOVL  XT,ACC
    MOVZ  AR0,AL
    MOVZ  AR1,AL
    MOVZ  AR2,AL
    MOVZ  AR3,AL
    MOVZ  AR4,AL
    MOVZ  AR5,AL
    MOVZ  AR6,AL
    MOVZ  AR7,AL
    MOVW  DP, #0

;------------------------------------------------
;   Restore ST0 and ST1.  Note OBJMODE is
;   the only bit not restored to its reset state.
;   OBJMODE is left set for C28x object operating
;   mode.
;
;  ST0 = 0x0000     ST1 = 0x0A0B
;  15:10 OVC = 0    15:13      ARP = 0
;   9: 7  PM = 0       12       XF = 0
;      6   V = 0       11  M0M1MAP = 1
;      5   N = 0       10  reserved
;      4   Z = 0        9  OBJMODE = 1
;      3   C = 0        8    AMODE = 0
;      2  TC = 0        7 IDLESTAT = 0
;      1 OVM = 0        6   EALLOW = 0
;      0 SXM = 0        5     LOOP = 0
;                       4      SPA = 0
;                       3     VMAP = 1
;                       2    PAGE0 = 0
;                       1     DBGM = 1
;                       0     INTM = 1
;-----------------------------------------------

    MOV  *SP++,#0
    MOV  *SP++,#0x0A0B
    POP  ST1
    POP  ST0

;------------------------------------------------
;   Jump to the EntryAddr as defined by the
;   boot mode selected and continue execution
;-----------------------------------------------

    LRETR