Hi, intermediate report:
We found a bug in isohdpfx.bin which spoils the C/H/S address conversion for BIOSes which do not support LBA addressing. http://www.syslinux.org/archives/2017-March/025692.html It turned out that David Christensen's BIOS does not support INT 13 AH 41 which causes isohdpfx.bin to use C/H/S addressing. If one forces this C/H/S addressing on qemu-system-i386, then the MBR reads the wrong blocks because it confuses heads-per-cylinder and sectors-per-head. Martin Str|mberg posted a patch: ----------------------------------------------------------------------- --- a/mbr/isohdpfx.S +++ b/mbr/isohdpfx.S @@ -48,8 +48,8 @@ isolinux_start_hybrid = 0x7c00+64+4 stack = 0x7c00 partoffset = (stack-8) driveno = (stack-14) -heads = (stack-16) -sectors = (stack-18) +sectors = (stack-16) +heads = (stack-18) ebios_flag = (stack-20) secpercyl = (stack-24) ----------------------------------------------------------------------- which with his qemu BIOS repairs C/H/S addressing. Regrettably this patch did not reach David Christensen, because he unsubscribed at [email protected] before the bug was found. It is not sure whether this really fixes the problem with David's BIOS because other than qemu's BIOS it did not read a wrong block content out of the test image but rather a block with all zeros. (The test image has its blocks filled with their address number. All zero is not among them, because at block 0 sits the MBR.) I am Cc-ing David with this mail in the hope that he will test the patch and report its outcome. ------------------------------------------------------------------- David: If it is the decisive trick, then patching the fixed MBR into the first 432 bytes of the ISO should make it bootable. If it is patched onto the start of the block_seq image on stick, then it should report block content that is not all zero, but rather 00 00 20 FC ... 00 00 20 FC If it does not solve the problem, then i would be interested in learning which block content is read after the patch. I.e. in this case please try again with Martin's diagnostic MBR with the patch applied and let me know the diagnostic output. Have a nice day :) Thomas
