ref: f8935b5778397074d41a48205e5c7f87d7b531fe
dir: purgatorio/os/boot/pc/mbr.s
/* * Hard disc boot block. Loaded at 0x7C00, relocates to 0x0600: * 8a mbr.s; 8l -o mbr -l -H3 -T0x0600 mbr.8 */ #include "x16.h" #include "mem.h" /*#define FLOPPY 1 /* test on a floppy */ #define TRACE(C) PUSHA;\ CLR(rBX);\ MOVB $C, AL;\ LBI(0x0E, rAH);\ BIOSCALL(0x10);\ POPA /* * We keep data on the stack, indexed by BP. */ #define Xdap 0x00 /* disc address packet */ #define Xtable 0x10 /* partition table entry */ #define Xdrive 0x12 /* starting disc */ #define Xtotal 0x14 /* sum of allocated data above */ /* * Start: loaded at 0000:7C00, relocate to 0000:0600. * Boot drive is in rDL. */ TEXT _start(SB), $0 CLI CLR(rAX) MTSR(rAX, rSS) /* 0000 -> rSS */ LWI((0x7C00-Xtotal), rSP) /* 7Bxx -> rSP */ MW(rSP, rBP) /* set the indexed-data pointer */ MTSR(rAX, rDS) /* 0000 -> rDS, source segment */ LWI(0x7C00, rSI) /* 7C00 -> rSI, source offset */ MTSR(rAX, rES) /* 0000 -> rES, destination segment */ LWI(0x600, rDI) /* 0600 -> rDI, destination offset */ LWI(0x100, rCX) /* 0100 -> rCX, loop count (words) */ CLD REP; MOVSL /* MOV DS:[(E)SI] -> ES:[(E)DI] */ FARJUMP16(0x0000, _start0600(SB)) TEXT _start0600(SB), $0 #ifdef FLOPPY LBI(0x80, rDL) #else CLRB(rAL) /* some systems pass 0 */ CMPBR(rAL, rDL) JNE _save LBI(0x80, rDL) #endif /* FLOPPY */ _save: SXB(rDL, Xdrive, xBP) /* save disc */ LWI(confidence(SB), rSI) /* for that warm, fuzzy feeling */ CALL16(BIOSputs(SB)) LWI(_start+0x01BE(SB), rSI) /* address of partition table */ LWI(0x04, rCX) /* 4 entries in table */ LBI(0x80, rAH) /* active entry value */ CLRB(rAL) /* inactive entry value */ _activeloop0: LXB(0x00, xSI, rBL) /* get active entry from table */ CMPBR(rBL, rAH) /* is this an active entry? */ JEQ _active CMPBR(rBL, rAL) /* if not active it should be 0 */ JNE _invalidMBR ADDI(0x10, rSI) /* next table entry */ DEC(rCX) JNE _activeloop0 LWI(noentry(SB), rSI) CALL16(buggery(SB)) _active: MW(rSI, rDI) /* save table address */ _activeloop1: ADDI(0x10, rSI) /* next table entry */ DEC(rCX) JEQ _readsector LXB(0x00, xSI, rBL) /* get active entry from table */ CMPBR(rBL, rAH) /* is this an active entry? */ JNE _activeloop1 /* should only be one active */ _invalidMBR: LWI(invalidMBR(SB), rSI) CALL16(buggery(SB)) _readsector: LBI(0x41, rAH) /* check extensions present */ LWI(0x55AA, rBX) LXB(Xdrive, xBP, rDL) /* drive */ BIOSCALL(0x13) /* CF set on failure */ JCS _readsector2 CMPI(0xAA55, rBX) JNE _readsector2 ANDI(0x0001, rCX) JEQ _readsector2 _readsector42: SBPBI(0x10, Xdap+0) /* packet size */ SBPBI(0x00, Xdap+1) /* reserved */ SBPBI(0x01, Xdap+2) /* number of blocks to transfer */ SBPBI(0x00, Xdap+3) /* reserved */ SBPWI(0x7C00, Xdap+4) /* transfer buffer :offset */ SBPWI(0x0000, Xdap+6) /* transfer buffer seg: */ LXW(0x08, xDI, rAX) /* LBA (64-bits) */ SBPW(rAX, Xdap+8) LXW(0x0A, xDI, rAX) SBPW(rAX, Xdap+10) SBPWI(0x0000, Xdap+12) SBPWI(0x0000, Xdap+14) MW(rBP, rSI) /* disk address packet */ LBI(0x42, rAH) /* extended read */ BIOSCALL(0x13) /* CF set on failure */ JCC _readsectorok LWI(ioerror(SB), rSI) CALL16(buggery(SB)) /* * Read a sector from a disc using the traditional BIOS call. * For BIOSCALL(0x13/AH=0x02): * rAH 0x02 * rAL number of sectors to read (1) * rCH low 8 bits of cylinder * rCL high 2 bits of cylinder (7-6), sector (5-0) * rDH head * rDL drive * rES:rBX buffer address */ _readsector2: LXB(0x01, xDI, rDH) /* head */ LXW(0x02, xDI, rCX) /* save active cylinder/sector */ LWI(0x0201, rAX) /* read one sector */ LXB(Xdrive, xBP, rDL) /* drive */ LWI(0x7C00, rBX) /* buffer address (rES already OK) */ BIOSCALL(0x13) /* CF set on failure */ JCC _readsectorok LWI(ioerror(SB), rSI) CALL16(buggery(SB)) _readsectorok: LWI(0x7C00, rBX) /* buffer address (rES already OK) */ LXW(0x1FE, xBX, rAX) CMPI(0xAA55, rAX) JNE _bbnotok /* * Jump to the loaded PBS. * rDL and rSI should still contain the drive * and partition table pointer respectively. */ MW(rDI, rSI) FARJUMP16(0x0000, 0x7C00) _bbnotok: LWI(invalidPBS(SB), rSI) TEXT buggery(SB), $0 CALL16(BIOSputs(SB)) LWI(reboot(SB), rSI) CALL16(BIOSputs(SB)) _wait: CLR(rAX) /* wait for any key */ BIOSCALL(0x16) _reset: CLR(rBX) /* set ES segment for BIOS area */ MTSR(rBX, rES) LWI(0x0472, rBX) /* warm-start code address */ LWI(0x1234, rAX) /* warm-start code */ POKEW /* MOVW AX, ES:[BX] */ FARJUMP16(0xFFFF, 0x0000) /* reset */ /* * Output a string to the display. * String argument is in rSI. */ TEXT BIOSputs(SB), $0 PUSHA CLR(rBX) _BIOSputs: LODSB ORB(rAL, rAL) JEQ _BIOSputsret LBI(0x0E, rAH) BIOSCALL(0x10) JMP _BIOSputs _BIOSputsret: POPA RET /* "No active entry in MBR" */ TEXT noentry(SB), $0 BYTE $'N'; BYTE $'o'; BYTE $' '; BYTE $'a'; BYTE $'c'; BYTE $'t'; BYTE $'i'; BYTE $'v'; BYTE $'e'; BYTE $' '; BYTE $'e'; BYTE $'n'; BYTE $'t'; BYTE $'r'; BYTE $'y'; BYTE $' '; BYTE $'i'; BYTE $'n'; BYTE $' '; BYTE $'M'; BYTE $'B'; BYTE $'R'; BYTE $'\z'; /* "Invalid MBR" */ TEXT invalidMBR(SB), $0 BYTE $'I'; BYTE $'n'; BYTE $'v'; BYTE $'a'; BYTE $'l'; BYTE $'i'; BYTE $'d'; BYTE $' '; BYTE $'M'; BYTE $'B'; BYTE $'R'; BYTE $'\z'; /* "I/O error" */ TEXT ioerror(SB), $0 BYTE $'I'; BYTE $'/'; BYTE $'O'; BYTE $' '; BYTE $'e'; BYTE $'r'; BYTE $'r'; BYTE $'o'; BYTE $'r'; BYTE $'\z'; /* "Invalid PBS" */ TEXT invalidPBS(SB), $0 BYTE $'I'; BYTE $'n'; BYTE $'v'; BYTE $'a'; BYTE $'l'; BYTE $'i'; BYTE $'d'; BYTE $' '; BYTE $'P'; BYTE $'B'; BYTE $'S'; BYTE $'\z'; /* "\r\nPress almost any key to reboot..." */ TEXT reboot(SB), $0 BYTE $'\r';BYTE $'\n'; BYTE $'P'; BYTE $'r'; BYTE $'e'; BYTE $'s'; BYTE $'s'; BYTE $' '; BYTE $'a'; BYTE $'l'; BYTE $'m'; BYTE $'o'; BYTE $'s'; BYTE $'t'; BYTE $' '; BYTE $'a'; BYTE $'n'; BYTE $'y'; BYTE $' '; BYTE $'k'; BYTE $'e'; BYTE $'y'; BYTE $' '; BYTE $'t'; BYTE $'o'; BYTE $' '; BYTE $'r'; BYTE $'e'; BYTE $'b'; BYTE $'o'; BYTE $'o'; BYTE $'t'; BYTE $'.'; BYTE $'.'; BYTE $'.'; BYTE $'\z'; /* "MBR..." */ TEXT confidence(SB), $0 BYTE $'M'; BYTE $'B'; BYTE $'R'; BYTE $'.'; BYTE $'.'; BYTE $'.'; BYTE $'\z';