[apple/boot.git] / i386 / doc / README

/* 
 * Mach Operating System
 * Copyright (c) 1990 Carnegie-Mellon University
 * Copyright (c) 1989 Carnegie-Mellon University
 * All rights reserved.  The CMU software License Agreement specifies
 * the terms and conditions for use and redistribution.
 */

		AT386 Protected Mode Bootstrap Loader
		=====================================

1. Overview of Startup
   -------------------

	After the system is rebooted, the BIOS bootstrap routine reads Sector
	1, Track 0 into memory at location 0000:7C00H.  If location 0000:7DFEH
	(last two bytes of that sector) contains the value AA55H, the BIOS
	bootstrap routine will transfer control to 0000:7C00H.  Otherwise, the
	boot code in that sector is bad and the boot routine stops.
	
	For DOS compatibility reason, one extra stage of boot is required if
	the boot device is a hard disk.  The first sector of the hard disk will
	contain the MOS-DOS boot code and a boot record partition table.
	When this sector is loaded into 0000:7C00H, it will relocate itself
	to somewhere else and then load the first sector of the active
	partition into 0000:7C00H.  Both UNIX and DOS use the command "fdisk"
	to install this first sector into the hard disk and to manipulate
	the hard disk partitions.

	
2. The First Stage Bootstrap Loader
   --------------------------------

	After startup, the first stage boot is loaded at 0000:7C00H.  This
	first stage boot will load itself and the second stage boot into
	memory at location 0000:1000H.  For floppy disks, the first cylinder
	is reserved as the boot cylinder, and the boot code (first and second)
	will be loaded from there.  Currently, only double sided, high density
	(15 sectors per track) floppies are supported.  For hard disks, the
	first 29 sectors of the active partition is reserved for boot code
	which will be loaded by the first stage boot.  All the disk types
	recognized by BIOS are supported by this bootstrap loader. 


3. The Second Stage Bootstrap Loader
   --------------------------------

	After the boot code is loaded, the control is passed to the second
	stage bootstrap loader "boot2()".  In order to be able to load the
	big kernel image (bigger than 512K or 640K, depends on the memory
	configuration), the second stage boot loader will run on the protected
	mode.  This bootstarp loader does not have any stand alone device
	drivers, all the I/O's are through the BIOS calls.  Since the first
	stage boot code will no longer be used at this moment, the memory
	location of the first stage boot code (0000:1000H to 0000:1200H) will
	be used as an internal buffer for BIOS calls.  Immediately after this 
	internal buffer is the GDT table for the second stage boot loader. 
	Since this boot loader needs to switch back and forth between protected
	and real mode in order to use BIOS calls, the limit of the boot code
	and boot data segments must not be greater than 64K.
	
	The boot loader loads the kernel image at memory location above 1 MB
	to skip the memory hole between 521K/640K and 1MB.  After the kernel
	is loaded, the boot loader stores the information in the stack and
	then passes control to kernel.  Currently, the three information passed
	fromm the boot loader to the kernel are type of the boot device, size
	of the base memory and size of the extended memory.


4. The UNIX Startup
   ----------------

	Since the boot loader loads the kernel image at memory location above
	1MB, the kernel has to start as protected mode.  In addition, the
	link editor description file (vuifile) has to indicate that
	the text and data segments start above 1MB.  Also, the boot loader
	passes the infomation to the kernel through the stack.


5. Disk Layout and Bad Block Handling
   ---------------------------------
	
    The System V/386 Release 3.2 (AT) disk layout will be used as the disk
    layout for the MACH System on the AT platform.

    This disk layout is as follows:

	* Reserve the first sector of cylinder 0 for the DOS boot record which
	  contains the master boot code (446 bytes) and the partition table.
	  (Refer to DOS Technical Reference Manual page 9-6 to 9-10).

	* Reserve the first 29 sectors of the UNIX partition for the first
	  and the second stage bootstrap.

	* Reserve the 30th sector of the UNIX partition for the pdinfo and
	  the vtoc tables.

	* Reserve the 31st to the 34th sectors of the UNIX partition for the
	  bad track and the bad block mapping tables.

	* Reserve up to 253 consecutive tracks when required, beginning with
	  the 35th sector of the UNIX partition, for alternate tracks.

	* Reserve up to 253 consecutive blocks, beginning with the first
	  sector after the alternate tracks area, for alternate blocks.

	 SEC
	  1
	----------------------------------------------------
	| X |                                              | CYL 0, TRK 0
	----------------  ..........    --------------------
	|                 ..........                       |
	----------------  ..........    --------------------
	|                 ..........                       |
   ===============================================================
    ^   |               BOOTSTRAP                          | CYL N, TRK M
    |   ----------------------------------------------------
    |   |                              |30 |31 |32 |33 |34 |
	----------------------------------------------------  ---
    U   |                 ..........                       |   ^
    N   ----------------  ..........   ---------------------   |
    I   |                 ..........                       | Alternate Tracks
    X   ----------------  ..........   ---------------------   |
        |                 ..........                       |   V
    P   ----------------------------------------------------  --- 
    A   |                 ..........                       |   ^
    R   ----------------  ..........   ---------------------   |
    T   |                 ..........                       | Alternate Blocks
    I   ----------------  ..........   --------------------    |
    T   |                 ..........                       |   V
    I   ----------------------------------------------------  ---
    O   |  Unix root partition starts from here            |
    N   ----------------                   -----------------
  	|                                                  |
        ----------------------------------------------------
        |                                                  |
	----------------------------------------------------
	|                                                  |
    |   ---------------------------------------------------
    |   |                                                  |
    |   ----------------------------------------------------
    V   |                                                  |
   ===============================================================
  	|                   ........                       |
	---------------     ........          --------------
	|                   ........                       |
	----------------------------------------------------


	The bad block handling mechanism is as follows:

	* Use the alternate track in the alternate tracks area if the
	  track containing the target sector is bad.

	* Use the alternate block in the alternate blocks area if the
	  target sector is bad.


6. How to make:
   -----------

	Since the kernel image is loaded above 1 MB, the kernel must start
	as protected mode.  This means that this bootstrap loader will work
	only when the corresponding changes on the kernel startup code are done.

	The make command to generate this bootstrap loader is:

	make -f boot.mk fdboot      (floppy boot loader)
	make -f boot.mk hdboot      (wini boot loader)
Commit	Line	Data
14c7c974 A	1	/*
	2	* Mach Operating System
	3	* Copyright (c) 1990 Carnegie-Mellon University
	4	* Copyright (c) 1989 Carnegie-Mellon University
	5	* All rights reserved. The CMU software License Agreement specifies
	6	* the terms and conditions for use and redistribution.
	7	*/
	8
	9	AT386 Protected Mode Bootstrap Loader
	10	=====================================
	11
	12	1. Overview of Startup
	13	-------------------
	14
	15	After the system is rebooted, the BIOS bootstrap routine reads Sector
	16	1, Track 0 into memory at location 0000:7C00H. If location 0000:7DFEH
	17	(last two bytes of that sector) contains the value AA55H, the BIOS
	18	bootstrap routine will transfer control to 0000:7C00H. Otherwise, the
	19	boot code in that sector is bad and the boot routine stops.
	20
	21	For DOS compatibility reason, one extra stage of boot is required if
	22	the boot device is a hard disk. The first sector of the hard disk will
	23	contain the MOS-DOS boot code and a boot record partition table.
	24	When this sector is loaded into 0000:7C00H, it will relocate itself
	25	to somewhere else and then load the first sector of the active
	26	partition into 0000:7C00H. Both UNIX and DOS use the command "fdisk"
	27	to install this first sector into the hard disk and to manipulate
	28	the hard disk partitions.
	29
	30
	31
	32	2. The First Stage Bootstrap Loader
	33	--------------------------------
	34
	35	After startup, the first stage boot is loaded at 0000:7C00H. This
	36	first stage boot will load itself and the second stage boot into
	37	memory at location 0000:1000H. For floppy disks, the first cylinder
	38	is reserved as the boot cylinder, and the boot code (first and second)
	39	will be loaded from there. Currently, only double sided, high density
	40	(15 sectors per track) floppies are supported. For hard disks, the
	41	first 29 sectors of the active partition is reserved for boot code
	42	which will be loaded by the first stage boot. All the disk types
	43	recognized by BIOS are supported by this bootstrap loader.
	44
	45
	46
	47	3. The Second Stage Bootstrap Loader
	48	--------------------------------
	49
	50	After the boot code is loaded, the control is passed to the second
	51	stage bootstrap loader "boot2()". In order to be able to load the
	52	big kernel image (bigger than 512K or 640K, depends on the memory
	53	configuration), the second stage boot loader will run on the protected
	54	mode. This bootstarp loader does not have any stand alone device
	55	drivers, all the I/O's are through the BIOS calls. Since the first
	56	stage boot code will no longer be used at this moment, the memory
	57	location of the first stage boot code (0000:1000H to 0000:1200H) will
	58	be used as an internal buffer for BIOS calls. Immediately after this
	59	internal buffer is the GDT table for the second stage boot loader.
	60	Since this boot loader needs to switch back and forth between protected
	61	and real mode in order to use BIOS calls, the limit of the boot code
	62	and boot data segments must not be greater than 64K.
	63
	64	The boot loader loads the kernel image at memory location above 1 MB
65	to skip the memory hole between 521K/640K and 1MB. After the kernel
66	is loaded, the boot loader stores the information in the stack and
67	then passes control to kernel. Currently, the three information passed
68	fromm the boot loader to the kernel are type of the boot device, size
69	of the base memory and size of the extended memory.
70
71
72	4. The UNIX Startup
73	----------------
74
75	Since the boot loader loads the kernel image at memory location above
76	1MB, the kernel has to start as protected mode. In addition, the
77	link editor description file (vuifile) has to indicate that
78	the text and data segments start above 1MB. Also, the boot loader
79	passes the infomation to the kernel through the stack.
80
81
82	5. Disk Layout and Bad Block Handling
83	---------------------------------
84
85	The System V/386 Release 3.2 (AT) disk layout will be used as the disk
86	layout for the MACH System on the AT platform.
87
88	This disk layout is as follows:
89
90	* Reserve the first sector of cylinder 0 for the DOS boot record which
91	contains the master boot code (446 bytes) and the partition table.
92	(Refer to DOS Technical Reference Manual page 9-6 to 9-10).
93
94	* Reserve the first 29 sectors of the UNIX partition for the first
95	and the second stage bootstrap.
96
97	* Reserve the 30th sector of the UNIX partition for the pdinfo and
98	the vtoc tables.
99
100	* Reserve the 31st to the 34th sectors of the UNIX partition for the
101	bad track and the bad block mapping tables.
102
103	* Reserve up to 253 consecutive tracks when required, beginning with
104	the 35th sector of the UNIX partition, for alternate tracks.
105
106	* Reserve up to 253 consecutive blocks, beginning with the first
107	sector after the alternate tracks area, for alternate blocks.
108
109	SEC
110	1
111	----------------------------------------------------
112	\| X \| \| CYL 0, TRK 0
113	---------------- .......... --------------------
114	\| .......... \|
115	---------------- .......... --------------------
116	\| .......... \|
117	===============================================================
118	^ \| BOOTSTRAP \| CYL N, TRK M
119	\| ----------------------------------------------------
120	\| \| \|30 \|31 \|32 \|33 \|34 \|
121	---------------------------------------------------- ---
122	U \| .......... \| ^
123	N ---------------- .......... --------------------- \|
124	I \| .......... \| Alternate Tracks
125	X ---------------- .......... --------------------- \|
126	\| .......... \| V
127	P ---------------------------------------------------- ---
128	A \| .......... \| ^
129	R ---------------- .......... --------------------- \|
130	T \| .......... \| Alternate Blocks
131	I ---------------- .......... -------------------- \|
132	T \| .......... \| V
133	I ---------------------------------------------------- ---
134	O \| Unix root partition starts from here \|
135	N ---------------- -----------------
136	\| \|
137	----------------------------------------------------
138	\| \|
139	----------------------------------------------------
140	\| \|
141	\| ---------------------------------------------------
142	\| \| \|
143	\| ----------------------------------------------------
144	V \| \|
145	===============================================================
146	\| ........ \|
147	--------------- ........ --------------
148	\| ........ \|
149	----------------------------------------------------
150
151
152	The bad block handling mechanism is as follows:
153
154	* Use the alternate track in the alternate tracks area if the
155	track containing the target sector is bad.
156
157	* Use the alternate block in the alternate blocks area if the
158	target sector is bad.
159
160
161
162
163	6. How to make:
164	-----------
165
166	Since the kernel image is loaded above 1 MB, the kernel must start
167	as protected mode. This means that this bootstrap loader will work
168	only when the corresponding changes on the kernel startup code are done.
169
170	The make command to generate this bootstrap loader is:
171
172	make -f boot.mk fdboot (floppy boot loader)
173	make -f boot.mk hdboot (wini boot loader)