WTF? Its supposed to be instrinsic. I check the assembler listing and I see the resulting "pop" op-code, yet when the program links it gives the following error: *** WARNING L1: UNRESOLVED EXTERNAL SYMBOL SYMBOL: __POP_ MODULE: BLA BLA BLA NOTE: The "Intrins.h" file is included (obviously), and preceeding the _pop_(.), I'm using _push_(.), ...yet push does not produce the error. Info: IDE-Version: µVision2 V2.40a Copyright (c) Keil Elektronik GmbH / Keil Software, Inc. 1995 - 2003 Tool Version Numbers: Toolchain Path: C:\Keil\C51\BIN\ C Compiler: C51.Exe V7.20 Assembler: A51.Exe V7.09 Linker/Locator: BL51.Exe V5.11 Librarian: LIB51.Exe V4.24 Hex Converter: OH51.Exe V2.6 CPU DLL: S8051.DLL V2.43 Dialog DLL: DP51.DLL V2.43