So XFDBOOT must load MS-DOS kernel (IO.SYS), DOS-C kernel or any other Free DOS kernel. I assume that unmodified IO.SYS and MSDOS.SYS files are disposed at the same places where MS-DOS utility SYS.COM put them. XFDBOOT utility MKBOOT.EXE only replace original MS-DOS boot sector with own boot sector. We can't load IPL.SYS without any modifications because it assumes that complete kernel image is loaded at 2000h:0, not at 70h:0. We must add loader leading sector before original DOS-C kernel image which place it at correct address. You may do it by following way: copy/b krnlhead.sys+ipl.sys a:\xipl.sys Now even fragmented XIPL.SYS will be loaded. Other kernels also must contain special loader leading sector. You may include unique code for concrete kernel to KRNLOAD.INC. Current demo version of KRNLOAD.INC is destined for loading DOS-C v. 0.90 kernel IPL.SYS. More about KRNLHEAD.ASM. This is source code of KRNLHEAD.SYS - loader leading sector. This sector will be loaded at 70h:0. Code contained in this sector ask BIOS for size of conventional memory and put kernel image sector map to the top of it. Sector map will be generated during kernel FAT-chain scanning, so kernel file may be fragmented. Any entry of kernel map contain CX and DX values for int 13h Read Sector call. Then code fragment between labels LOADER_BEG and LOADER_END (upper loader) will be moved just before kernel map, stack will be switched to last memory segment and control will be passed to upper loader. BUGS: Currently XFDBOOT Boot Sector code does not redefine Floppy Drive Parameter Table, and can load kernel only from standard (unerstandable by BIOS) diskette. Next versions: Now operator must remember exact names of all kernels on disk. In following version of XBOOT I'll try to show him root directory files ;-). ------------------------------------------------------------------------------ APPENDIX A == MS-DOS Bootstrap Time Memory Map == 0000:0000 旼컴컴컴컴컴컴컴컴컴컴커 Interrupt Vector Table � � � � � � 0400 쳐컴컴컴컴컴컴컴컴컴컴캑 BIOS Data 0500 쳐컴컴컴컴컴컴컴컴컴컴캑 Directory window � � 0700 쳐컴컴컴컴컴컴컴컴컴컴캑 First 3 sectors of IO.SYS � � � � � � � � � � 0D00 읕컴컴컴컴컴컴컴컴컴컴켸 ... ... ... ... ... ... ... ... ... ... ... ... ... ... ^ Bootstrap Stack ^ 7C00 쳐컴컴컴컴컴컴컴컴컴컴캑 Boot sector � � 7E00 읕컴컴컴컴컴컴컴�55횫A켸 IO.SYS entry point: 70h:0 Startup Data: [SP] - far pointer to Int 1e (FD Parameter Table) interrupt table entry [SP+4] - Int 1e vector old value (also far pointer :) CH - Media Descriptor DL - Physical Drive AX:BX - Data Start Sector (usually IO.SYS begins here) counting from the begining of physical drive DS:SI - ??? Directory window at 0:500h must contain IO.SYS/MSDOS.SYS entries as first two entries ------------------------------------------------------------------------------ APPENDIX B == FD alt. kernel Bootstrap Time Memory Map == 0000:0000 旼컴컴컴컴컴컴컴컴컴컴커 Interrupt Vector Table � � � � � � 0400 쳐컴컴컴컴컴컴컴컴컴컴캑 BIOS Data 0500 쳐컴컴컴컴컴컴컴컴컴컴캑 Directory window � � 0700 쳐컴컴컴컴컴컴컴컴컴컴캑 *First* "leading" sector of � � kernel �----------------------� � � (First 3 sectors of IO.SYS) � � � � 0D00 읕컴컴컴컴컴컴컴컴컴컴켸 ... ... ... ... ... ... ... ... ... ... ... ... ... ... ^ Bootstrap Stack ^ 7C00 쳐컴컴컴컴컴컴컴컴컴컴캑 Boot sector � � 7E00 쳐컴컴컴컴컴컴컴�55횫A캑 XBoot Dialogue Routines � XB � 8000 읕컴컴컴컴컴컴컴컴컴컴켸 ... ... ... ... ... ... ... ... ... ... ... ... ... ... XXXX:0000 旼컴컴컴컴컴컴컴컴컴컴커 Last 64 k segment (usually at 9000h) ... ... ... ... ... ... ... ... ... ... ... ... ... ... � � � � � � � � � � � � � � Upper loader stack 쳐컴컴컴컴컴컴컴컴컴컴캑 ^ ^ 쳐컴컴컴컴컴컴컴컴컴컴캑 "Upper" loader (very small:) XXXX:FFF0 읕컴컴컴컴컴컴컴컴컴컴켸 Kernel sector map (created by loader) Loader entry point: 70h:0 Startup Data: coincide with IO.SYS startup data. Upper loader must to prepare all registers and data after loading kernel image (and maybe to do all fixups) and then jump to kernel entry point. ------------------------------------------------------------------------------ Some features of 1.10 version. Most changes caused by trying to make loader compatible with Windows'95 IO.SYS kernel. Now XFDBOOT (and FDAK - for now unpublished, but wait a little) use following registers: ; ; Kernel Initialization. ; Expected register values: ; ; DL : Boot physical drive ; AX:BX : Long number of hidden sectors ; CH : Media descriptor of Boot drive ; CS : Kernel Segment * ; SI : 1st memory pragraph for DOS management or -1 * ; DI : Memory boundary - 1st non-DOS paragraph > ; DI : Starting kernel cluster ; CL : EXE-version DOS Flag (for traditional bootstrap 0) > ; BP : Pointer to Boot Sector ; Removed register usage lines marked by '*', and new lines marked by '>' signs. SI and DI registers were reserved in fact for ROM versions, but (as you can see) such scheme conflicts with Windows'95 one. Now XFDBOOT use BP, which points to Boot Sector, exactly before Boot Sector is situated reserved sectors before booted partition dword, and then follows area for stack. During kernel startup interrupt vector of int 1e and address of it (i.e. 0:78h) are pushed to stack. DI contains starting cluster of kernel: 0:7c00 [BP] 0:7bfe [BP-2] == AX 0:7bfc [BP-4] == BX Int 1e vector Int 1e vector addr ------------------------------------------------------------------------------