单片机启动文件.s有什么作用？

$NOMOD51;------------------------------------------------------------------------------;  This file is part of the C51 Compiler package;  Copyright (c) 1988-2005 Keil Elektronik GmbH and Keil Software, Inc.;  Version 8.01;;  *** <<< Use Configuration Wizard in Context Menu >>> ***;------------------------------------------------------------------------------;  STARTUP.A51:  This code is executed after processor reset.;;  To translate this file use A51 with the following invocation:;;     A51 STARTUP.A51;;  To link the modified STARTUP.OBJ file to your application use the following;  Lx51 invocation:;;     Lx51 your object file list, STARTUP.OBJ  controls;;------------------------------------------------------------------------------;;  User-defined  Power-On Initialization of Memory;;  With the following EQU statements the initialization of memory;  at processor reset can be defined:;;  IDATALEN: IDATA memory size <0x0-0x100>;      Note: The absolute start-address of IDATA memory is always 0;            The IDATA space overlaps physically the DATA and BIT areas.IDATALEN        EQU     80H;;  XDATASTART: XDATA memory start address <0x0-0xFFFF> ;      The absolute start address of XDATA memoryXDATASTART      EQU     0     ;;  XDATALEN: XDATA memory size <0x0-0xFFFF> ;      The length of XDATA memory in bytes.XDATALEN        EQU     0      ;;  PDATASTART: PDATA memory start address <0x0-0xFFFF> ;      The absolute start address of PDATA memoryPDATASTART      EQU     0H;;  PDATALEN: PDATA memory size <0x0-0xFF> ;      The length of PDATA memory in bytes.PDATALEN        EQU     0H;;

;------------------------------------------------------------------------------;; Reentrant Stack Initialization;;  The following EQU statements define the stack pointer for reentrant;  functions and initialized it:;;  Stack Space for reentrant functions in the SMALL model.;   IBPSTACK: Enable SMALL model reentrant stack;      Stack space for reentrant functions in the SMALL model.IBPSTACK        EQU     0       ; set to 1 if small reentrant is used.;   IBPSTACKTOP: End address of SMALL model stack <0x0-0xFF>;      Set the top of the stack to the highest location.IBPSTACKTOP     EQU     0xFF +1     ; default 0FFH+1  ; ;;  Stack Space for reentrant functions in the LARGE model.      ;   XBPSTACK: Enable LARGE model reentrant stack;      Stack space for reentrant functions in the LARGE model.XBPSTACK        EQU     0       ; set to 1 if large reentrant is used.;   XBPSTACKTOP: End address of LARGE model stack <0x0-0xFFFF>;      Set the top of the stack to the highest location.XBPSTACKTOP     EQU     0xFFFF +1   ; default 0FFFFH+1 ; ;;  Stack Space for reentrant functions in the COMPACT model.    ;   PBPSTACK: Enable COMPACT model reentrant stack;      Stack space for reentrant functions in the COMPACT model.PBPSTACK        EQU     0       ; set to 1 if compact reentrant is used.;;    PBPSTACKTOP: End address of COMPACT model stack <0x0-0xFFFF>;      Set the top of the stack to the highest location.PBPSTACKTOP     EQU     0xFF +1     ; default 0FFH+1  ; ;