正常情况下MDK是不能同时支持51和AMM的，除非装两个，但是这样太麻烦了，这个修改方法很简单，已经用了很久了，不知道的可以试试。
首先先装ARM版，破解一下，然后把安装目录下的 TOOLS.INI 文件里面添加红字内容（最好备份下），注意路径
[UV2]
CDB0=UV4STC.CDB("STC")
ORGANIZATION="Microsoft"
NAME="Microsoft", "1"
EMAIL="1"
ARMSEL=1
USERTE=1
BOOK0="UV4RELEASE_NOTES.HTM" ("uVision Release Notes",GEN)
RTEPATH="D:KEIL05PACK"
DEVICE_BOOKS=
SocketPort=1969854680
[ARM]
PATH="D:KEIL05ARM"
VERSION=5.00
PATH1="C:Program FilesGNU Tools ARM Embedded4.7 2012q4"
TOOLPREFIX=arm-none-eabi-
CPUDLL0=SARM.DLL(TDRV0,TDRV5,TDRV6,TDRV9)                                                                 # Drivers for ARM7/9 devices
CPUDLL1=SARMCM3.DLL(TDRV1,TDRV2,TDRV3,TDRV4,TDRV5,TDRV7,TDRV8,TDRV10,TDRV11,TDRV12,TDRV13,TDRV14,TDRV15)  # Drivers for Cortex-M devices
CPUDLL2=SARMCR4.DLL(TDRV7)                                                                                # Drivers for Cortex-R4 devices
BOOK0="HLPRELEASE_NOTES.HTM" ("Release Notes",GEN)
BOOK1="HLPARMTOOLS.chm" ("Complete User's Guide Selection",C)
TDRV0=BINUL2ARM.DLL("ULINK2/ME ARM Debugger")
TDRV1=BINUL2CM3.DLL("ULINK2/ME Cortex Debugger")
TDRV2=BINAGDIRDI.DLL("RDI Interface Driver")
TDRV3=BINABLSTCM.dll("Altera Blaster Cortex Debugger")
TDRV4=BINlmidk-agdi.dll("Stellaris ICDI")
TDRV5=SignumSigUV3Arm.dll("Signum Systems JTAGjet")
TDRV6=SeggerJLTAgdi.dll("JTAG / J-TRACE ARM")
TDRV7=SeggerJL2CM3.dll("JTAG / J-TRACE Cortex")
TDRV8=BINULP2CM3.DLL("ULINK Pro Cortex Debugger")
TDRV9=BINULP2ARM.DLL("ULINK Pro ARM Debugger")
TDRV10=NULinkNu_Link.dll("NULink Debugger")
TDRV11=SiLabsSLAB_CM_Keil.dll("SiLabs UDA Debugger")
TDRV12=STLinkST-LINKIII-KEIL_SWO.dll ("ST-Link Debugger")
TDRV13=BINCMSIS_AGDI.dll("CMSIS-DAP Debugger")
TDRV14=BINDbgFM.DLL("Fast Models Debugger")
TDRV15=PEMicroPemicro_ArmCortexInterface.dll("PEMicro Debugger")
LIC0=TIV74-FXUYI-GI05Q-NP6N4-0EQBH-JEH8Y
[ARMADS]
PATH="D:KEIL05ARM"
PATH1=".ARMCCin"
CPUDLL0=SARM.DLL(TDRV0,TDRV5,TDRV6,TDRV9)                                                                 # Drivers for ARM7/9 devices
CPUDLL1=SARMCM3.DLL(TDRV1,TDRV2,TDRV3,TDRV4,TDRV5,TDRV7,TDRV8,TDRV10,TDRV11,TDRV12,TDRV13,TDRV14,TDRV15)  # Drivers for Cortex-M devices
CPUDLL2=SARMCR4.DLL(TDRV7)                                                                                # Drivers for Cortex-R4 devices
BOOK0="HLPRELEASE_NOTES.HTM" ("Release Notes",GEN)
BOOK1="HLPARMTOOLS.chm" ("Complete User's Guide Selection",C)
TDRV0=BINUL2ARM.DLL("ULINK2/ME ARM Debugger")
TDRV1=BINUL2CM3.DLL("ULINK2/ME Cortex Debugger")
TDRV2=BINAGDIRDI.DLL("RDI Interface Driver")
TDRV3=BINABLSTCM.dll("Altera Blaster Cortex Debugger")
TDRV4=BINlmidk-agdi.dll("Stellaris ICDI")
TDRV5=SignumSigUV3Arm.dll("Signum Systems JTAGjet")
TDRV6=SeggerJLTAgdi.dll("JTAG / J-TRACE ARM")
TDRV7=SeggerJL2CM3.dll("JTAG / J-Trace Cortex")
TDRV8=BINULP2CM3.DLL("ULINK Pro Cortex Debugger")
TDRV9=BINULP2ARM.DLL("ULINK Pro ARM Debugger")
TDRV10=NULinkNu_Link.dll("NULink Debugger")
TDRV11=SiLabsSLAB_CM_Keil.dll("SiLabs UDA Debugger")
TDRV12=STLinkST-LINKIII-KEIL_SWO.dll ("ST-Link Debugger")
TDRV13=BINCMSIS_AGDI.dll("CMSIS-DAP Debugger")
TDRV14=BINDbgFM.DLL("Fast Models Debugger")
TDRV15=PEMicroPemicro_ArmCortexInterface.dll("PEMicro Debugger")
RTOS0=Dummy.DLL("Dummy")
RTOS1=VARTXARM.DLL ("RTX Kernel")

[C51]
PATH="D:KEIL05C51"" "
SN=K1DZP-5IUSH-A01UE  
VERSION=V9.01  
BOOK0="HLPRELEASE_NOTES.HTM" ("Release Notes",GEN)
BOOK1="HLPGS51.PDF" ("uVision2 Getting Started",GEN)
BOOK2="HLPC51.PDF" ("C51 User's Guide",GEN)
BOOK3="HLPC51LIB.CHM" ("C51 Library Functions",C)
BOOK4="HLPA51.PDF" ("Assembler/Utilities",GEN)
BOOK5="HLPTR51.CHM" ("RTX51 Tiny User's Guide",GEN)
BOOK6="HLPDBG51.CHM" ("uVision2 Debug Commands",GEN)
BOOK7="HLPISD51.CHM" ("ISD51 In System Debugger",GEN)
BOOK8="HLPFlashMon51.CHM" ("Flash Monitor",GEN)
BOOK9="MON390MON390.HTM" ("MON390: Dallas Contiguous Mode Monitor",GEN)
TDRV0=BINMON51.DLL ("Keil Monitor-51 Driver")  
TDRV1=BINISD51.DLL ("Keil ISD51 In-System Debugger")  
TDRV2=BINMON390.DLL ("MON390: Dallas Contiguous Mode")  
TDRV3=BINLPC2EMP.DLL ("LPC900 EPM Emulator/Programmer")  
TDRV4=BINUL2UPSD.DLL ("ST-uPSD ULINK Driver")  
RTOS1=RTXTINY.DLL ("RTX-51 Tiny")  
RTOS2=RTX51.DLL ("RTX-51 Full")  
LIC0=76PCX-HP74U-IRIXW-QEPXN-32TWY-CALVY


[C251]
BOOK0="HLPRELEASE_NOTES.HTM" ("Release Notes",GEN)
BOOK1="HLPGS51.PDF" ("uVision2 Getting Started",GEN)
BOOK2="HLPC251.PDF" ("C251 User's Guide",GEN)
BOOK3="HLPC251LIB.CHM" ("C251 Library Functions",C)
BOOK4="HLPA51.PDF" ("Assembler/Utilities",GEN)
BOOK5="HLPDBG251.CHM" ("uVision2 Debug Commands",GEN)
TDRV0=BINMON251.DLL ("Keil Monitor-251 Driver")
VERSION=V3.5

修改后保存，然后把keil51的安装文件夹下的C51文件夹复制到ARM版的安装目录下，然后选择器件里就有51系列单片机了

之后再去STC官网下个STC.CDB文件复制到安装目录下的UV4文件夹，就变成这样了


这样想建立ARM工程就选上面那个，建立51的工程就选下面那个，或者ARM工程里也有AT的51单片机可选。我试过了，4.12版本以上的都可以用，我的是5.0版本MDK。用起来比装两个MDK方便多了