Loaded at 7C00h, setup stack and copy block from 7C00 to 7E00h. Push 7E20h on the stack and return near which will begin execution at 7E20h. This is the initial code loaded at 07C0:0000 and the disassembled code relocated at 07E0:0000 continues below.<\/p>\n
After relocation the entry is at 7E20, I left out the 7E00 to 7E1F code which is not used. I am really only interested in the basic follow and the loaded and follow-on module, so some of the commenting could be better. In general, a Boot Manager partition is looked for and a simple consistency check is done on the MBR.<\/p>\n
If you see below a check is made to verify INT 13 Extensions API support (see CheckINT13Ext<\/a>). The result is stored at 3000:0000, if supported 58333149h is stored if not 0 is stored. This value is used later.<\/p>\n ReadDrive<\/a> procedure is used to load the second drive without using the INT 13 Extensions API, but is used later to load the partition boot information using INT 13 Extensions API.<\/p>\n Things for my own note, there are 3 possible error messages while processing the LVM MBR: SYS01462, SYS01463, and SYS01464. Also, the drives look like they must support INT 13 Extensions API which should not be a problem now days.<\/p>\n The partition boot information is loaded at 7C00 and execution is continued.<\/p>\n <\/p>\n <\/p>\n After the code is moved from 0000:7C00 to 0000:7E00 the entry point is 0000:7E20.<\/p>\n <\/p>\n <\/p>\n There are 3 possible error messages while processing the LVM MBR: SYS01462, SYS01463, and SYS01464:<\/p>\n <\/p>\n <\/p>\n ReadDrive procedure loads MBR of the second drive and the boot record of the botable partition:<\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Loaded at 7C00h, setup stack and copy block from 7C00 to 7E00h. Push 7E20h on the stack and return near which will begin execution at 7E20h. This is the initial code loaded at 07C0:0000 and the disassembled code relocated at 07E0:0000 continues below. After relocation the entry is at 7E20, I left out the 7E00…<\/p>\n(7C00h)\n_entry\t\tproc near\n\t; disable interupts\n\tcli\n\t; setup stack\n\tmov\tax, 30h\n\tmov\tss, ax\n\tmov\tsp, 100h ; decimal 256\n\t; enable interupts\n\tsti\n\t; move 7C00 to 7E00 +512\n\tcld\n\txor\tax, ax ; Zero out the Accumulator\n\tmov\tds, ax ; Zero-out Data Segment\n\tmov\tes, ax ; Zero-out Extra Segment\n\tmov\tsi, 7C00h ; Copy from here...\n\tmov\tdi, 7E00h ; copy to here: 0000:7E00\n\tmov\tcx, 200h ; 200h (512 words) count\n\trep movsw\n\t; push return addr 7E20 and execute return\n\tpush\t7E20h\n\tretn\n_entry\t\tendp<\/pre>\n<\/div>\n
(7E20)\n_relocentry proc near\n\tmov si, 7EFAh ; si point to SYS01462 message start\n\tmov bx, 7FBEh ; bx set to first partition info location\n; *** Check for Boot Manager\n; read each MBR record and check if it is type Boot Manager (0x0A).\nbootman00:\n\tcmp byte ptr [bx+4], 0Ah ; cmp part type 11 (0Ah) BootManager\n\tjz short checktype ; jump if Boot Manager partition found\n\tadd bx, 10h ; setup to read next partition entry\n\tcmp bx, 7FFEh ; check for sig - end of MBR if not get next\n\tjl short bootman00\n\txor ax, ax ; zero ax\n\tint 13h ; DISK - RESET DISK SYSTEM\n\t; DISK - GET DRIVE PARAMETERS (PC,XT286,CONV,PS,ESDI,SCSI)\n\t; AH = 08h\n\t; DL = drive (bit 7 set for hard disk)\n\t; Return:CF set on error\n\t; AH = status (07h) (see #0211)\n\t; CF clear if successful\n\t; AH = 00h\n\t; AL = 00h on at least some BIOSes\n\t; BL = drive type (AT\/PS2 floppies only) (see #0219)\n\t; CH = low eight bits of maximum cylinder number\n\t; CL = maximum sector number (bits 5-0)\n\t; high two bits of maximum cylinder number (bits 7-6)\n\t; DH = maximum head number\n\t; DL = number of drives\n\t; ES:DI -> drive parameter table (floppies only)\n\t; check for drive #2\n\tmov ah, 8\n\tmov dl, 81h ; drive 2\n\tint 13h\n\tjb short checktype ; jump no 2nd drive\n\t; read 2nd drive MBR\n\tmov cx, 7FB4h\n\tmov dl, 81h\n\tcall ReadDrive\n\tor ah, ah\n\tjnz short checktype ; jump if read error\n\tcmp word ptr ds:7DFEh, 0AA55h ; check signature\n\tjnz short checktype ; jump not valid MBR signature\n\tmov bx, 7DBEh ; set start 2nd drive MBR info\n\t; Check for Boot Manager on 2nd drive\nbootman01:\n\tcmp byte ptr [bx+4], 0Ah\n\tjnz short bootman02\n\t; the following get executed if a 0Ah found on 2nd drive\n\tmov dl, 81h ; second drive 81h in dl\n\tmov cx, bx ; bx - location of 0Ah partition info\n\tjmp short extcheck2\nbootman02:\n\tadd bx, 10h\n\tcmp bx, 7DFEh\n\tjl short bootman01\n; *** Check for bootable partition\n; get here if boot manager part found on 1st drive, no second drive, and falls through\n; no boot manager found on second drive\nchecktype:\n\tmov bx, 7FBEh ; load _bootind partition 1\n\txor cx, cx ; zero cx\n; seems to just run through the MBR and ensure it is somewhat correct\nchecktype1:\n\tcmp byte ptr [bx], 80h ; is it bootable?\n\tjnz short checktype2 ; jump if not bootable\n\tor cx, cx\n\tjnz short DispMsgEntry ; Not zero display SYS01462 error and hang\n\tmov cx, bx\n\tjmp short checktype3\nchecktype2:\n\tcmp byte ptr [bx], 0 ; is it 0 (not-bootable) - so if not 80h or 0 then unknown _bootind\n\tjnz short DispMsgEntry ; Not zero display SYS01462 error and hang\nchecktype3:\n\tadd bx, 10h ; increment to next partition record\n\tcmp bx, 7FFEh ; at the end of MBR - check signature\n\tjl short checktype1\n\tor cx, cx\n\tjnz short extcheck1\n\tint 18h ; None were bootable, so start ROM-BASIC many\n\t\t\t; BIOS simply display \"PRESS A KEY TO REBOOT\"\n\t\t\t; when an Interrupt 18h is executed.\nextcheck1:\n\tmov dl, 80h ; first drive\n; at this point dl contains drive number 80h or 81h\nextcheck2:\n\tpusha ; PUSH AX, CX, DX, BX, SP, BP, SI and DI\n\tcall CheckINT13Ext\n\tpopa ; POP AX, CX, DX, BX, SP, BP, SI and DI\n\tpush dx\n\tpush cx\n\tcall ReadDrive\n\tjz short vbr00 ; jump no error\n\tmov si, 7F0Fh ; SYS01463 Message\n\tjmp short DispMsgEntry ; Not zero display error and hang\nvbr00:\n\tmov si, 7F24h ; SYS01464 Message\n\tcmp ds:SigEnd, 0AA55h ; compare to end block signature\n\tjnz short DispMsgEntry ; Not zero display error and hang\n\tpop si\t; seems to hold MBR pointer\n\tpop dx\t; boot drive number\n\tjmp far ptr 0000:7C00h ; ** jump 0000:7C00 **\n_relocentry endp\n; IBM\/MS INT 13 Extensions - INSTALLATION CHECK\n; AH = 41h\n; BX = 55AAh\n; DL = drive (80h-FFh)\n; Return:CF set on error (extensions not supported)\n; AH = 01h (invalid function)\n; CF clear if successful\n; BX = AA55h if installed\n; AH = major version of extensions\n; 01h = 1.x\n; 20h = 2.0 \/ EDD-1.0\n; 21h = 2.1 \/ EDD-1.1\n; 30h = EDD-3.0\n; AL = internal use\n; CX = API subset support bitmap (see #0248)\n; DH = extension version (v2.0+ ??? -- not present in 1.x)\n; Note: The Phoenix Enhanced Disk Drive Specification v1.0 uses version 2.0 of the INT 13 Extensions API\n;\n; See Also: AH=42h\"INT 13 Ext\" - AH=48h\"INT 13 Ext\"\n;\n; Bitfields for IBM\/MS INT 13 Extensions API support bitmap:\n;\n; Bit(s) Description (Table 0248)\n; 0 extended disk access functions (AH=42h-44h,47h,48h) supported\n; 1 removable drive controller functions (AH=45h,46h,48h,49h,INT 15\/AH=52h)\n; supported\n; 2 enhanced disk drive (EDD) functions (AH=48h,AH=4Eh) supported.\n; Extended drive parameter table is valid (see #0250,#0255)\n; 3-15 reserved (0)\n;\n; NOTE: From : http:\/\/lrs.uni-passau.de\/support\/doc\/interrupt-57\/RB-0668.HTM\n; checks for extended int 13 capability -\n; Exit if supported 3000:0000 move 58333149h\n; not supported mov 0\n; * dl contains drive number on entry\n(7EBA)\nCheckINT13Ext proc near\n\tmov ah, 41h\n\tmov bx, 55AAh\n\tint 13h\n\tjb short NoINT13Ext ; jump to NoINT13Ext is not supported\n\tcmp bx, 0AA55h ; AA55h if installed INT 13 Extensions API\n\tjnz short NoINT13Ext\n\tcmp ah, 21h ; major version of extensions 21h = 2.1 \/ EDD-1.1\n\tjb short NoINT13Ext\n\ttest cl, 1 ; Test if extended Disk Access functions supported\n\tjz short NoINT13Ext\n\tmov eax, 58333149h\n\tjmp short INT13Continue\nNoINT13Ext:\n\txor ax, ax ; zero ax if ext 13 not supported\nINT13Continue:\n\tpush 3000h ; store eax at 3000:0000\n\tpop fs\n\tmov fs:0, eax\n\tretn\nCheckINT13Ext endp<\/pre>\n<\/div>\n
***** DISPLAY MESSAGE LOOP *****\n(7EE8)\nDispMsgEntry:\n\txor bx, bx\t; zero bx\n\tjmp short DispMsg\nDispNext:\n\tint 10h\nDispMsg:\n\tmov ah, 0Eh\n\tlodsb\t\t; get char of message\n\tor al, al\t; check if end\n\tjnz short DispNext\n\tsti\nHangLoop:\n\tjmp short HangLoop\n; The following boot message information is from Bob Eager,\n; Tavi Systems page http:\/\/www.tavi.co.uk\/os2pages\/boot.html:\n; SYS01462\n; The partition table on the startup drive is incorrect. Generally, this\n; means either that more than one partition is marked active, or one of\n; the partitions has a status byte with a value other than 00H or 80H,\n; which are the only legal values.\n7EF8 db 12h\n7EF9 db 0\n7EFA _SYS01462 db 'OS\/2 !! SYS01462',0Dh,0Ah,0\n; SYS01463\n; The operating system cannot be loaded from the startup drive. This is\n; caused by a disk read error, while reading the boot sector of the\n; active partition\n7F0D db 12h\n7F0E db 0\n7F0F _SYS01463 db 'OS\/2 !! SYS01463',0Dh,0Ah,0\n; SYS01464\n; The operating system is missing from the startup drive. A valid boot\n; sector for a partition should contain the values 055H and 0AAH in its\n; last two bytes, in that order. This is a simple validation check,\n; intended to prevent attempts to boot from a corrupt or unformatted\n; partition. This message is generated if the validation check for\n; these two bytes fails.\n7F22 db 12h\n7F23 db 0\n7F24 _SYS01464 db 'OS\/2 !! SYS01464',0Dh,0Ah,0<\/pre>\n<\/div>\n
; On entry:\n; DL == drive\n; CX == location of Track + Sector to read\n; location 3000:0000 == 49h ext read supported else old 0 - 1023 read\nReadDrive proc near\n\tmov bx, cx\n\tmov di, 5\n\t ; see INT13Ext storage 3000:0000\n\tpush 3000h\n\tpop fs\n\tcmp byte ptr fs:0, 49h\n\tjz short ExtRead ; equals 49h if ext supported\n\t; CX contains both the cylinder number (10 bits, possible\n\t; values are 0 to 1023) and the sector number (6 bits, possible values are 1 to 63):\n\tmov cx, [bx+2]\n\t; Head\n\tmov dh, [bx+1]\n\t; (ES):BX = Memory Buffer\n\tmov bx, 7C00h\nReadOld01:\n\txor ax, ax\n\tint 13h ; Reset DISK\n\tmov ax, 201h ; Function 2 AH == 00000010 \/ Sectors To Read Count AL == 00000001\n\tint 13h ; INT 13, -- Read Sectors From Drive\n\tjnb short ReadOld02 ; error reading\n\tdec di ; number of retries - default 5\n\tjg short ReadOld01 ; retry read\nReadOld02:\n\tretn\n\t; The following are the \"INT 13 Extensions Installation Check\" and\n\t; the Extended READ sectors from Hard Drive (Function 42h) routines.\n\t; **** Normal entry if Ext int13 supported after CheckINT13Ext call\nExtRead:\n\t; dl == drive number\n\tpush ds\n\tmov eax, [bx+8] ; load number of sectors before partition from MBR\n\t; set fs and ds 0x3000\n\tpush fs\n\tpop ds\n\t; DS:SI \tsegment:offset pointer to the DAP<\/a>, see below\n\t; DAP == 3000:0008\n\tmov si, 8\n\t; DAP : Disk Address Packet (16 bytes)\n\t; offset range \tsize \tdescription\n\t; 00h \t\t1 byte \tsize of DAP = 16 = 10h\n\t; 01h \t\t1 byte \tunused, should be zero\n\t; 02h \t\t1 byte \tnumber of sectors to be read, 0..127 (= 7Fh)\n\t; 03h \t\t1 byte \tunused, should be zero\n\t; 04h..07h 4 bytes \tsegment:offset pointer to the memory buffer to which sectors will be transferred\n\t; 08h..0Fh 8 bytes \tabsolute number of the start of the sectors to be read (1st sector of drive has number 0)\n\tmov ds:4, eax\n\tmov [si+8], eax\n\txor eax, eax ; zero eax\n\tmov word ptr [si], 10h ; 00h BYTE 10h (size of packet)\n\tmov word ptr [si+2], 1 ; number of blocks to transfer\n\tmov word ptr [si+4], 7C00h ; -> transfer buffer\n\tmov [si+6], ax\n\tmov [si+0Ch], eax\n\t; DAP\"\n\t; 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10\n\t; 10 00 01 00 [00 7C 00 00] [00 3F 00 00 00 00 00 00]\nReadJmp04:\n\tsub ax, ax\n\tint 13h ; Reset drive\n\tmov ah, 42h ; function number for extended read\n\tint 13h\n\tjnb short ReadJmp05 ; Error\n\tdec di\n\tja short ReadJmp04 ; Retry read\nReadJmp05:\n\tpop ds\n\tretn\nReadDrive endp<\/pre>\n<\/div>\n
; The following is an example using my current Virtual PC drive\n; src\/jfs\/utils\/libfs\/mbr.h -- from openJFS source located on Netlabs.org\n;\n; struct part {\n; UCHAR bootind; \/* 0x80 means partition is bootable *\/\n; UCHAR starthead; \/* head number of partition start *\/\n; UCHAR startsect; \/* sector number *\/\n; UCHAR startcyl; \/* cylinder number *\/\n; UCHAR systind; \/* partition ID *\/\n; UCHAR endhead; \/* head number of partition end *\/\n; UCHAR endsect; \/* sector number *\/\n; UCHAR endcyl; \/* cylinder number *\/\n; ULONG lsn; \/* number of sectors before partition *\/\n; ULONG nsects; \/* number of sectors in partition *\/\n; };\n;\n; struct mbr {\n; UCHAR code[0x1be]; \/* boot record code and data *\/\n; struct part ptbl[4]; \/* the partition table *\/\n; USHORT sig; \/* special signature *\/\n; };\n7FB8 OptiDiskSig dd 0\n7FBC dw 0CC33h\n7FBE ; ***** Partition 1 *****\n7FBE _bootind db 80h ; bootable\n7FBF _starthead db 1\n7FC0 _startsect db 1\n7FC1 _startcyl db 0\n7FC2 _systind db 7\n7FC3 _endhead db 3Fh\n7FC4 _endsect db 0FFh\n7FC5 _endcyl db 0F6h\n7FC6 _lsn dd 3Fh\n7FCA _nsects dd 3E7201h\n7FCE ; ***** Partition 2 *****\n7FCE Part2 db 10h dup(0)\n7FDE ; ***** Partition 3 *****\n7FDE Part3 db 10h dup(0)\n7FEE ; ***** Partition 4 *****\n7FEE Part4 db 10h dup(0)\n7FFE ; ***** Signature *****\n7FFE SigEnd dw 0AA55h<\/pre>\n<\/div>\n