diff --git a/Visual Studio Projects/ID16.vcproj b/Visual Studio Projects/ID16.vcproj
index 657c94d1..3bd4c0f3 100644
--- a/Visual Studio Projects/ID16.vcproj	
+++ b/Visual Studio Projects/ID16.vcproj	
@@ -41,7 +41,7 @@
 				Name="VCCLCompilerTool"
 				Optimization="0"
 				AdditionalIncludeDirectories="./;../;../slirp;../slirp_glue;../slirp_glue/qemu;../slirp_glue/qemu/win32/include;../../windows-build/include;;../../windows-build/include/SDL2"
-				PreprocessorDefinitions="_CRT_NONSTDC_NO_WARNINGS;SIM_BUILD_TOOL=simh-Visual-Studio-Project;_CRT_NONSTDC_NO_WARNINGS;_CRT_SECURE_NO_WARNINGS;_WINSOCK_DEPRECATED_NO_WARNINGS;SIM_NEED_GIT_COMMIT_ID;HAVE_PCRE_H;PCRE_STATIC;HAVE_LIBEDIT"
+				PreprocessorDefinitions="IFP_IN_MEM;_CRT_NONSTDC_NO_WARNINGS;SIM_BUILD_TOOL=simh-Visual-Studio-Project;_CRT_SECURE_NO_WARNINGS;_WINSOCK_DEPRECATED_NO_WARNINGS;SIM_NEED_GIT_COMMIT_ID;HAVE_PCRE_H;PCRE_STATIC;HAVE_LIBEDIT"
 				KeepComments="false"
 				BasicRuntimeChecks="0"
 				RuntimeLibrary="1"
@@ -127,7 +127,7 @@
 				OmitFramePointers="true"
 				WholeProgramOptimization="true"
 				AdditionalIncludeDirectories="./;../;../slirp;../slirp_glue;../slirp_glue/qemu;../slirp_glue/qemu/win32/include;../../windows-build/include;;../../windows-build/include/SDL2"
-				PreprocessorDefinitions="_CRT_NONSTDC_NO_WARNINGS;SIM_BUILD_TOOL=simh-Visual-Studio-Project;_CRT_NONSTDC_NO_WARNINGS;_CRT_SECURE_NO_WARNINGS;_WINSOCK_DEPRECATED_NO_WARNINGS;SIM_NEED_GIT_COMMIT_ID;HAVE_PCRE_H;PCRE_STATIC;HAVE_LIBEDIT"
+				PreprocessorDefinitions="IFP_IN_MEM;_CRT_NONSTDC_NO_WARNINGS;SIM_BUILD_TOOL=simh-Visual-Studio-Project;_CRT_SECURE_NO_WARNINGS;_WINSOCK_DEPRECATED_NO_WARNINGS;SIM_NEED_GIT_COMMIT_ID;HAVE_PCRE_H;PCRE_STATIC;HAVE_LIBEDIT"
 				StringPooling="true"
 				RuntimeLibrary="0"
 				EnableFunctionLevelLinking="true"
diff --git a/descrip.mms b/descrip.mms
index 482ef691..3534466e 100644
--- a/descrip.mms
+++ b/descrip.mms
@@ -490,7 +490,7 @@ ID16_SOURCE = $(ID16_DIR)ID16_CPU.C,$(ID16_DIR)ID16_SYS.C,$(ID16_DIR)ID_DP.C,\
               $(ID16_DIR)ID_IO.C,$(ID16_DIR)ID_LP.C,$(ID16_DIR)ID_MT.C,\
               $(ID16_DIR)ID_PAS.C,$(ID16_DIR)ID_PT.C,$(ID16_DIR)ID_TT.C,\
               $(ID16_DIR)ID_UVC.C,$(ID16_DIR)ID16_DBOOT.C,$(ID16_DIR)ID_TTP.C
-ID16_OPTIONS = /INCL=($(SIMH_DIR),$(ID16_DIR))/DEF=($(CC_DEFS))
+ID16_OPTIONS = /INCL=($(SIMH_DIR),$(ID16_DIR))/DEF=($(CC_DEFS),"IFP_IN_MEM=1")
 
 #
 # Interdata 32-bit CPU.
diff --git a/makefile b/makefile
index 084f7f08..dd076015 100644
--- a/makefile
+++ b/makefile
@@ -1925,7 +1925,7 @@ ID16 = ${ID16D}/id16_cpu.c ${ID16D}/id16_sys.c ${ID16D}/id_dp.c \
 	${ID16D}/id_fd.c ${ID16D}/id_fp.c ${ID16D}/id_idc.c ${ID16D}/id_io.c \
 	${ID16D}/id_lp.c ${ID16D}/id_mt.c ${ID16D}/id_pas.c ${ID16D}/id_pt.c \
 	${ID16D}/id_tt.c ${ID16D}/id_uvc.c ${ID16D}/id16_dboot.c ${ID16D}/id_ttp.c
-ID16_OPT = -I ${ID16D}
+ID16_OPT = -DIFP_IN_MEM -I ${ID16D}
 
 
 ID32D = ${SIMHD}/Interdata