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/*
2 
 * FAT Partition Boot Sector. Loaded at 0x7C00:
3 
 *	8a pbslba.s; 8l -o pbslba -l -H3 -T0x7C00 pbslba.8
4 
 * Will load the target at LOADSEG*16+LOADOFF, so the target
5 
 * should be probably be loaded with LOADOFF added to the
6 
 * -Taddress.
7 
 * If LOADSEG is a multiple of 64KB and LOADOFF is 0 then
8 
 * targets larger than 64KB can be loaded.
9 
 *
10 
 * This code is uses Enhanced BIOS Services for Disc Drives and
11 
 * can be used with discs up to 137GB in capacity.
12 
 *
13 
 * It relies on the _volid field in the FAT header containing
14 
 * the LBA of the root directory.
15 
 */
16 
#include "x16.h"
17 
 
18 
#define LOADSEG		(0x10000/16)	/* where to load code (64KB) */
19 
#define LOADOFF		0
20 
#define DIROFF		0x0200		/* where to read the root directory */
21 
 
22 
/*
23 
 * FAT directory entry.
24 
 */
25 
#define Dname		0x00
26 
#define Dnamesz	0x0B
27 
#define Dext		0x08
28 
#define Dattr		0x0B
29 
#define Dtime		0x16
30 
#define Ddate		0x18
31 
#define Dstart		0x1A
32 
#define Dlengthlo	0x1C
33 
#define Dlengthhi	0x1E
34 
 
35 
#define Dirsz		0x20
36 
 
37 
/*
38 
 * Data is kept on the stack, indexed by rBP.
39 
 */
40 
#define Xdap		0x00		/* disc address packet */
41 
#define Xrootsz		0x10		/* file data area */
42 
#define Xdrive		0x12		/* boot drive, passed by BIOS or MBR */
43 
#define Xtotal		0x14		/* sum of allocated data above */
44 
 
45 
TEXT _magic(SB), $0
46 
	BYTE $0xEB; BYTE $0x3C;		/* jmp .+ 0x3C  (_start0x3E) */
47 
	BYTE $0x90			/* nop */
48 
TEXT _version(SB), $0
49 
	BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
50 
	BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00
51 
TEXT _sectsize(SB), $0
52 
	BYTE $0x00; BYTE $0x00
53 
TEXT _clustsize(SB), $0
54 
	BYTE $0x00
55 
TEXT _nresrv(SB), $0
56 
	BYTE $0x00; BYTE $0x00
57 
TEXT _nfats(SB), $0
58 
	BYTE $0x00
59 
TEXT _rootsize(SB), $0
60 
	BYTE $0x00; BYTE $0x00
61 
TEXT _volsize(SB), $0
62 
	BYTE $0x00; BYTE $0x00
63 
TEXT _mediadesc(SB), $0
64 
	BYTE $0x00
65 
TEXT _fatsize(SB), $0
66 
	BYTE $0x00; BYTE $0x00
67 
TEXT _trksize(SB), $0
68 
	BYTE $0x00; BYTE $0x00
69 
TEXT _nheads(SB), $0
70 
	BYTE $0x00; BYTE $0x00
71 
TEXT _nhiddenlo(SB), $0
72 
	BYTE $0x00; BYTE $0x00
73 
TEXT _nhiddenhi(SB), $0
74 
	BYTE $0x00; BYTE $0x00;
75 
TEXT _bigvolsize(SB), $0
76 
	BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
77 
TEXT _driveno(SB), $0
78 
	BYTE $0x00
79 
TEXT _reserved0(SB), $0
80 
	BYTE $0x00
81 
TEXT _bootsig(SB), $0
82 
	BYTE $0x00
83 
TEXT _volid(SB), $0
84 
	BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
85 
TEXT _label(SB), $0
86 
	BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
87 
	BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00
88 
	BYTE $0x00; BYTE $0x00; BYTE $0x00
89 
TEXT _type(SB), $0
90 
	BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
91 
	BYTE $0x00; BYTE $0x00; BYTE $0x00; BYTE $0x00;
92 
 
93 
_start0x3E:
94 
	CLI
95 
	CLR(rAX)
96 
	MTSR(rAX, rSS)			/* 0000 -> rSS */
97 
	MTSR(rAX, rDS)			/* 0000 -> rDS, source segment */
98 
	MTSR(rAX, rES)
99 
	LWI(_magic-Xtotal(SB), rSP)
100 
	MW(rSP, rBP)			/* set the indexed-data pointer */
101 
 
102 
	SBPB(rDL, Xdrive)		/* save the boot drive */
103 
 
104 
	/* booting from a CD starts us at 7C0:0.  Move to 0:7C00 */
105 
	PUSHR(rAX)
106 
	LWI(_nxt(SB), rAX)
107 
	PUSHR(rAX)
108 
	BYTE $0xCB	/* FAR RET */
109 
 
110 
TEXT _nxt(SB), $0
111 
	STI
112 
 
113 
	LWI(confidence(SB), rSI)	/* for that warm, fuzzy feeling */
114 
	CALL16(BIOSputs(SB))
115 
 
116 
	LBI(0x41, rAH)			/* check extensions present */
117 
	LWI(0x55AA, rBX)
118 
	LXB(Xdrive, xBP, rDL)		/* drive */
119 
	BIOSCALL(0x13)			/* CF set on failure */
120 
	JCS _jmp01
121 
	CMPI(0xAA55, rBX)
122 
	JNE _jmp01
123 
	ANDI(0x0001, rCX)
124 
	JEQ _jmp01
125 
 
126 
					/* rCX contains 0x0001 */
127 
	SBPWI(0x0010, Xdap+0)		/* reserved + packet size */
128 
	SBPW(rCX, Xdap+2)		/* reserved + # of blocks to transfer */
129 
 
130 
	DEC(rCX)
131 
	SBPW(rCX, Xdap+12)
132 
	SBPW(rCX, Xdap+14)
133 
 
134 
	CALL16(dreset(SB))
135 
 
136 
_jmp00:
137 
	LW(_volid(SB), rAX)		/* Xrootlo */
138 
	LW(_volid+2(SB), rDX)		/* Xroothi */
139 
 
140 
	LWI(_magic+DIROFF(SB), rBX)
141 
	CALL16(BIOSread(SB))		/* read the root directory */
142 
 
143 
	LWI((512/Dirsz), rBX)
144 
 
145 
	LWI(_magic+DIROFF(SB), rDI)	/* compare first directory entry */
146 
 
147 
_cmp00:
148 
	PUSHR(rDI)			/* save for later if it matches */
149 
	LWI(bootfile(SB), rSI)
150 
	LWI(Dnamesz, rCX)
151 
	REP
152 
	CMPSB
153 
	POPR(rDI)
154 
	JEQ _jmp02
155 
 
156 
	DEC(rBX)
157 
	JEQ _jmp01
158 
 
159 
	ADDI(Dirsz, rDI)
160 
	JMP _cmp00
161 
_jmp01:
162 
	CALL16(buggery(SB))
163 
 
164 
_jmp02:
165 
	CLR(rBX)			/* a handy value */
166 
	LW(_rootsize(SB), rAX)		/* calculate and save Xrootsz */
167 
	LWI(Dirsz, rCX)
168 
	MUL(rCX)
169 
	LW(_sectsize(SB), rCX)
170 
	PUSHR(rCX)
171 
	DEC(rCX)
172 
	ADD(rCX, rAX)
173 
	ADC(rBX, rDX)
174 
	POPR(rCX)			/* _sectsize(SB) */
175 
	DIV(rCX)
176 
	PUSHR(rAX)			/* Xrootsz */
177 
 
178 
	/*
179 
	 * rDI points to the matching directory entry.
180 
	 */
181 
	LXW(Dstart, xDI, rAX)		/* starting sector address */
182 
	DEC(rAX)			/* that's just the way it is */
183 
	DEC(rAX)
184 
	LB(_clustsize(SB), rCL)
185 
	CLRB(rCH)
186 
	MUL(rCX)
187 
	LW(_volid(SB), rCX)		/* Xrootlo */
188 
	ADD(rCX, rAX)
189 
	LW(_volid+2(SB), rCX)		/* Xroothi */
190 
	ADC(rCX, rDX)
191 
	POPR(rCX)			/* Xrootsz */
192 
	ADD(rCX, rAX)
193 
	ADC(rBX, rDX)
194 
 
195 
	PUSHR(rAX)			/* calculate how many sectors to read */
196 
	PUSHR(rDX)
197 
	LXW(Dlengthlo, xDI, rAX)
198 
	LXW(Dlengthhi, xDI, rDX)
199 
	LW(_sectsize(SB), rCX)
200 
	PUSHR(rCX)
201 
	DEC(rCX)
202 
	ADD(rCX, rAX)
203 
	ADC(rBX, rDX)
204 
	POPR(rCX)			/* _sectsize(SB) */
205 
	DIV(rCX)
206 
	MW(rAX, rCX)
207 
	POPR(rDX)
208 
	POPR(rAX)
209 
 
210 
	LWI(LOADSEG, rBX)		/* address to load into (seg+offset) */
211 
	MTSR(rBX, rES)			/*  seg */
212 
	LWI(LOADOFF, rBX)		/*  offset */
213 
 
214 
_readboot:
215 
	CALL16(BIOSread(SB))		/* read the sector */
216 
 
217 
	LW(_sectsize(SB), rDI)		/* bump addresses/counts */
218 
	ADD(rDI, rBX)
219 
	JCC _incsecno
220 
 
221 
	MFSR(rES, rDI)			/* next 64KB segment */
222 
	ADDI(0x1000, rDI)
223 
	MTSR(rDI, rES)
224 
 
225 
_incsecno:
226 
	CLR(rDI)
227 
	INC(rAX)
228 
	ADC(rDI, rDX)
229 
	LOOP _readboot
230 
 
231 
	LWI(LOADSEG, rDI)		/* set rDS for loaded code */
232 
	MTSR(rDI, rDS)
233 
	FARJUMP16(LOADSEG, LOADOFF)	/* no deposit, no return */
234 
 
235 
TEXT buggery(SB), $0
236 
	LWI(error(SB), rSI)
237 
	CALL16(BIOSputs(SB))
238 
 
239 
_wait:
240 
	CLR(rAX)			/* wait for almost any key */
241 
	BIOSCALL(0x16)
242 
 
243 
_reset:
244 
	CLR(rBX)			/* set ES segment for BIOS area */
245 
	MTSR(rBX, rES)
246 
 
247 
	LWI(0x0472, rBX)		/* warm-start code address */
248 
	LWI(0x1234, rAX)		/* warm-start code */
249 
	POKEW				/* MOVW	AX, ES:[BX] */
250 
 
251 
	FARJUMP16(0xFFFF, 0x0000)	/* reset */
252 
 
253 
 
254 
/*
255 
 * Read a sector from a disc. On entry:
256 
 *   rDX:rAX	sector number
257 
 *   rES:rBX	buffer address
258 
 */
259 
TEXT BIOSread(SB), $0
260 
	LWI(5, rDI)			/* retry count (ATAPI ZIPs suck) */
261 
_retry:
262 
	PUSHA				/* may be trashed by BIOSCALL */
263 
 
264 
	SBPW(rBX, Xdap+4)		/* transfer buffer :offset */
265 
	MFSR(rES, rDI)			/* transfer buffer seg: */
266 
	SBPW(rDI, Xdap+6)
267 
	SBPW(rAX, Xdap+8)		/* LBA (64-bits) */
268 
	SBPW(rDX, Xdap+10)
269 
 
270 
	MW(rBP, rSI)			/* disk address packet */
271 
	LBI(0x42, rAH)			/* extended read */
272 
	LBPB(Xdrive, rDL)		/* form drive */
273 
	BIOSCALL(0x13)			/* CF set on failure */
274 
	JCC _BIOSreadret
275 
 
276 
	POPA
277 
	DEC(rDI)			/* too many retries? */
278 
	JEQ _ioerror
279 
 
280 
	CALL16(dreset(SB))
281 
	JMP _retry
282 
 
283 
_ioerror:
284 
	LWI(ioerror(SB), rSI)
285 
	CALL16(BIOSputs(SB))
286 
	JMP _wait
287 
 
288 
_BIOSreadret:
289 
	POPA
290 
	RET
291 
 
292 
TEXT dreset(SB), $0
293 
	PUSHA
294 
	CLR(rAX)			/* rAH == 0 == reset disc system */
295 
	LBPB(Xdrive, rDL)
296 
	BIOSCALL(0x13)
297 
	ORB(rAH, rAH)			/* status (0 == success) */
298 
	POPA
299 
	JNE _ioerror
300 
	RET
301 
 
302 
/*
303 
 * Output a string to the display.
304 
 * String argument is in rSI.
305 
 */
306 
TEXT BIOSputs(SB), $0
307 
	PUSHA
308 
	CLR(rBX)
309 
_BIOSputs:
310 
	LODSB
311 
	ORB(rAL, rAL)
312 
	JEQ _BIOSputsret
313 
 
314 
	LBI(0x0E, rAH)
315 
	BIOSCALL(0x10)
316 
	JMP _BIOSputs
317 
 
318 
_BIOSputsret:
319 
	POPA
320 
	RET
321 
 
322 
/* "Bad format or I/O error\r\nPress almost any key to reboot..." */
323 
TEXT error(SB), $0
324 
	BYTE $'B'; BYTE $'a'; BYTE $'d'; BYTE $' ';
325 
	BYTE $'f'; BYTE $'o'; BYTE $'r'; BYTE $'m';
326 
	BYTE $'a'; BYTE $'t'; BYTE $' '; BYTE $'o';
327 
	BYTE $'r'; BYTE $' ';
328 
/* "I/O error\r\nPress almost any key to reboot..." */
329 
TEXT ioerror(SB), $0
330 
	BYTE $'I'; BYTE $'/'; BYTE $'O'; BYTE $' ';
331 
	BYTE $'e'; BYTE $'r'; BYTE $'r'; BYTE $'o';
332 
	BYTE $'r'; BYTE $'\r';BYTE $'\n';
333 
	BYTE $'P'; BYTE $'r'; BYTE $'e'; BYTE $'s';
334 
	BYTE $'s'; BYTE $' '; BYTE $'a'; BYTE $' ';
335 
	BYTE $'k'; BYTE $'e'; BYTE $'y';
336 
	BYTE $' '; BYTE $'t'; BYTE $'o'; BYTE $' ';
337 
	BYTE $'r'; BYTE $'e'; BYTE $'b'; BYTE $'o';
338 
	BYTE $'o'; BYTE $'t';
339 
	BYTE $'.'; BYTE $'.'; BYTE $'.';
340 
	BYTE $'\z';
341 
 
342 
#ifdef USEBCOM
343 
/* "B       COM" */
344 
TEXT bootfile(SB), $0
345 
	BYTE $'B'; BYTE $' '; BYTE $' '; BYTE $' ';
346 
	BYTE $' '; BYTE $' '; BYTE $' '; BYTE $' ';
347 
	BYTE $'C'; BYTE $'O'; BYTE $'M';
348 
	BYTE $'\z';
349 
#else
350 
/* "9LOAD      " */
351 
TEXT bootfile(SB), $0
352 
	BYTE $'9'; BYTE $'L'; BYTE $'O'; BYTE $'A';
353 
	BYTE $'D'; BYTE $' '; BYTE $' '; BYTE $' ';
354 
	BYTE $' '; BYTE $' '; BYTE $' ';
355 
	BYTE $'\z';
356 
#endif /* USEBCOM */
357 
 
358 
/* "PBS..." */
359 
TEXT confidence(SB), $0
360 
	BYTE $'P'; BYTE $'B'; BYTE $'S'; BYTE $'2';
361 
	BYTE $'.'; BYTE $'.'; BYTE $'.';
362 
	BYTE $'\z';