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/*
* FAT Partition Boot Sector. Loaded at 0x7C00:
* 8a pbs.s; 8l -o pbs -l -H3 -T0x7C00 pbs.8
* Will load the target at LOADSEG*16+LOADOFF, so the target
* should be probably be loaded with LOADOFF added to the
* -Taddress.
* If LOADSEG is a multiple of 64KB and LOADOFF is 0 then
* targets larger than 64KB can be loaded.
*
* This code uses the traditional INT13 BIOS interface and can
* therefore only access the first 8.4GB of the disc.
*
* It relies on the _volid field in the FAT header containing
* the LBA of the root directory.
*/
#include "x16.h"
#include "mem.h"
#define LOADSEG (0x10000/16) /* where to load code (64KB) */
#define LOADOFF 0
#define DIROFF 0x0200 /* where to read the root directory */
/*
* FAT directory entry.
*/
#define Dname 0x00
#define Dext 0x08
#define Dattr 0x0B
#define Dtime 0x16
#define Ddate 0x18
#define Dstart 0x1A
#define Dlengthlo 0x1C
#define Dlengthhi 0x1E
#define Dirsz 0x20
/*
* Data is kept on the stack, indexed by rBP.
*/
#define Xdap 0x00 /* disc address packet */
#define Xrootsz 0x10 /* file data area */
#define Xdrive 0x12 /* boot drive, passed by BIOS or MBR */
#define Xtotal 0x14 /* sum of allocated data above */
TEXT _magic(SB), $0
BYTE $0xEB; BYTE $0x3C; /* jmp .+ 0x3C (_start0x3E) */
BYTE $0x90 /* nop */
TEXT _version(SB), $0
BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00
TEXT _sectsize(SB), $0
BYTE $0x00; BYTE $0x00
TEXT _clustsize(SB), $0
BYTE $0x00
TEXT _nresrv(SB), $0
BYTE $0x00; BYTE $0x00
TEXT _nfats(SB), $0
BYTE $0x00
TEXT _rootsize(SB), $0
BYTE $0x00; BYTE $0x00
TEXT _volsize(SB), $0
BYTE $0x00; BYTE $0x00
TEXT _mediadesc(SB), $0
BYTE $0x00
TEXT _fatsize(SB), $0
BYTE $0x00; BYTE $0x00
TEXT _trksize(SB), $0
BYTE $0x00; BYTE $0x00
TEXT _nheads(SB), $0
BYTE $0x00; BYTE $0x00
TEXT _nhiddenlo(SB), $0
BYTE $0x00; BYTE $0x00
TEXT _nhiddenhi(SB), $0
BYTE $0x00; BYTE $0x00;
TEXT _bigvolsize(SB), $0
BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
TEXT _driveno(SB), $0
BYTE $0x00
TEXT _reserved0(SB), $0
BYTE $0x00
TEXT _bootsig(SB), $0
BYTE $0x00
TEXT _volid(SB), $0
BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
TEXT _label(SB), $0
BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00
BYTE $0x00; BYTE $0x00; BYTE $0x00
TEXT _type(SB), $0
BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
_start0x3E:
CLI
CLR(rAX)
MTSR(rAX, rSS) /* 0000 -> rSS */
MTSR(rAX, rDS) /* 0000 -> rDS, source segment */
MTSR(rAX, rES)
LWI(_magic-Xtotal(SB), rSP)
MW(rSP, rBP) /* set the indexed-data pointer */
SBPB(rDL, Xdrive) /* save the boot drive */
/* booting from a CD starts us at 7C0:0. Move to 0:7C00 */
PUSHR(rAX)
LWI(_nxt(SB), rAX)
PUSHR(rAX)
BYTE $0xCB /* FAR RET */
TEXT _nxt(SB), $0
STI
LWI(confidence(SB), rSI) /* for that warm, fuzzy feeling */
CALL16(BIOSputs(SB))
CALL16(dreset(SB))
_jmp00:
LW(_volid(SB), rAX) /* Xrootlo */
LW(_volid+2(SB), rDX) /* Xroothi */
LWI(_magic+DIROFF(SB), rBX)
CALL16(BIOSread(SB)) /* read the root directory */
LWI((512/Dirsz), rBX)
LWI(_magic+DIROFF(SB), rDI) /* compare first directory entry */
_cmp00:
PUSHR(rDI) /* save for later if it matches */
LWI(bootfile(SB), rSI)
LWI(Dattr, rCX)
REP
CMPSB
POPR(rDI)
JEQ _jmp02
DEC(rBX)
JEQ _jmp01
ADDI(Dirsz, rDI)
JMP _cmp00
_jmp01:
CALL16(buggery(SB))
_jmp02:
CLR(rBX) /* a handy value */
LW(_rootsize(SB), rAX) /* calculate and save Xrootsz */
LWI(Dirsz, rCX)
MUL(rCX)
LW(_sectsize(SB), rCX)
PUSHR(rCX)
DEC(rCX)
ADD(rCX, rAX)
ADC(rBX, rDX)
POPR(rCX) /* _sectsize(SB) */
DIV(rCX)
PUSHR(rAX) /* Xrootsz */
/*
* rDI points to the matching directory entry.
*/
LXW(Dstart, xDI, rAX) /* starting sector address */
DEC(rAX) /* that's just the way it is */
DEC(rAX)
LB(_clustsize(SB), rCL)
CLRB(rCH)
MUL(rCX)
LW(_volid(SB), rCX) /* Xrootlo */
ADD(rCX, rAX)
LW(_volid+2(SB), rCX) /* Xroothi */
ADC(rCX, rDX)
POPR(rCX) /* Xrootsz */
ADD(rCX, rAX)
ADC(rBX, rDX)
PUSHR(rAX) /* calculate how many sectors to read */
PUSHR(rDX)
LXW(Dlengthlo, xDI, rAX)
LXW(Dlengthhi, xDI, rDX)
LW(_sectsize(SB), rCX)
PUSHR(rCX)
DEC(rCX)
ADD(rCX, rAX)
ADC(rBX, rDX)
POPR(rCX) /* _sectsize(SB) */
DIV(rCX)
MW(rAX, rCX)
POPR(rDX)
POPR(rAX)
LWI(LOADSEG, rBX) /* address to load into (seg+offset) */
MTSR(rBX, rES) /* seg */
LWI(LOADOFF, rBX) /* offset */
_readboot:
CALL16(BIOSread(SB)) /* read the sector */
LW(_sectsize(SB), rDI) /* bump addresses/counts */
ADD(rDI, rBX)
JCC _incsecno
MFSR(rES, rDI) /* next 64KB segment */
ADDI(0x1000, rDI)
MTSR(rDI, rES)
_incsecno:
CLR(rDI)
INC(rAX)
ADC(rDI, rDX)
LOOP _readboot
LWI(LOADSEG, rDI) /* set rDS for loaded code */
MTSR(rDI, rDS)
FARJUMP16(LOADSEG, LOADOFF) /* no deposit, no return */
TEXT buggery(SB), $0
LWI(error(SB), rSI)
CALL16(BIOSputs(SB))
_wait:
CLR(rAX) /* wait for almost 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 */
/*
* Read a sector from a disc. On entry:
* rDX:rAX sector number
* rES:rBX buffer address
* For BIOSCALL(0x13):
* 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
*/
TEXT BIOSread(SB), $0
LWI(5, rDI) /* retry count (ATAPI ZIPs suck) */
_retry:
PUSHA /* may be trashed by BIOSCALL */
PUSHR(rBX)
LW(_trksize(SB), rBX)
LW(_nheads(SB), rDI)
IMUL(rDI, rBX)
OR(rBX, rBX)
JZ _ioerror
_okay:
DIV(rBX) /* cylinder -> rAX, track,sector -> rDX */
MW(rAX, rCX) /* save cylinder */
ROLI(0x08, rCX) /* swap rC[HL] */
SHLBI(0x06, rCL) /* move high bits up */
MW(rDX, rAX)
CLR(rDX)
LW(_trksize(SB), rBX)
DIV(rBX) /* head -> rAX, sector -> rDX */
INC(rDX) /* sector numbers are 1-based */
ANDI(0x003F, rDX) /* should not be necessary */
OR(rDX, rCX)
MW(rAX, rDX)
SHLI(0x08, rDX) /* form head */
LBPB(Xdrive, rDL) /* form drive */
POPR(rBX)
LWI(0x0201, rAX) /* form command and sectors */
BIOSCALL(0x13) /* CF set on failure */
JCC _BIOSreadret
POPA
DEC(rDI) /* too many retries? */
JEQ _ioerror
CALL16(dreset(SB))
JMP _retry
_ioerror:
LWI(ioerror(SB), rSI)
CALL16(BIOSputs(SB))
JMP _wait
_BIOSreadret:
POPA
RET
TEXT dreset(SB), $0
PUSHA
CLR(rAX) /* rAH == 0 == reset disc system */
LBPB(Xdrive, rDL)
BIOSCALL(0x13)
ORB(rAH, rAH) /* status (0 == success) */
POPA
JNE _ioerror
RET
/*
* 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
/* "Bad format or I/O error\r\nPress almost any key to reboot..."*/
TEXT error(SB), $0
BYTE $'B'; BYTE $'a'; BYTE $'d'; BYTE $' ';
BYTE $'f'; BYTE $'o'; BYTE $'r'; BYTE $'m';
BYTE $'a'; BYTE $'t'; BYTE $' '; BYTE $'o';
BYTE $'r'; BYTE $' ';
/* "I/O error\r\nPress almost any key to reboot..." */
TEXT ioerror(SB), $0
BYTE $'I'; BYTE $'/'; BYTE $'O'; BYTE $' ';
BYTE $'e'; BYTE $'r'; BYTE $'r'; BYTE $'o';
BYTE $'r'; BYTE $'\r';BYTE $'\n';
BYTE $'P'; BYTE $'r'; BYTE $'e'; BYTE $'s';
BYTE $'s'; BYTE $' '; BYTE $'a'; 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';
#ifdef USEBCOM
/* "B COM" */
TEXT bootfile(SB), $0
BYTE $'B'; BYTE $' '; BYTE $' '; BYTE $' ';
BYTE $' '; BYTE $' '; BYTE $' '; BYTE $' ';
BYTE $'C'; BYTE $'O'; BYTE $'M';
BYTE $'\z';
#else
/* "9LOAD " */
TEXT bootfile(SB), $0
BYTE $'9'; BYTE $'L'; BYTE $'O'; BYTE $'A';
BYTE $'D'; BYTE $' '; BYTE $' '; BYTE $' ';
BYTE $' '; BYTE $' '; BYTE $' ';
BYTE $'\z';
#endif /* USEBCOM */
/* "PBS..." */
TEXT confidence(SB), $0
BYTE $'P'; BYTE $'B'; BYTE $'S'; BYTE $'1';
BYTE $'.'; BYTE $'.'; BYTE $'.';
BYTE $'\z';
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