[boot] Cleanup boot sector code, add boot documentation

Documentation/text/boot.txt

@@ -148,9 +148,79 @@ ELKS "INITSEG" (setup data segment) offsets:
 | 01F1 | 497 | byte | setup_sects       | size in 512-byte sects   | build.c
 | 01F2 | 498 | word | ROOTFLAGS         | UNUSED                   |
 | 01F4 | 500 | word | syssize           | kernel size in paras     | build.c
-| 01F6 | 502 | word | elks_flags        | BLOB, BIOS_DRV           | boot_sect.S
+| 01F6 | 502 | byte | elks_flags        | BLOB, BIOS_DRV           | boot_sect.S
+! 01F7 | 503 | byte |                   | UNUSED                   |
 | 01F8 | 504 | word | RAMDISK           | UNUSED                   |
 | 01FA | 506 | word | SVGAMODE          | UNUSED                   |
 | 01FC | 508 | word | root_dev          | BIOS drive or kdev_t     | build.c,boot_sect.S
 | 01FE | 510 | word | boot_flag         | AA55h                    | boot_sect.S
 +------+-----+------+-------------------+--------------------------+------------
+
+
+ELKS memory at boot and detailed boot description
+
+ +Step -----------------  seg  off  linear size (size  start) -------------------------------+
+ |  interrupt vectors    0000:0000 00000  0400 (1024) @0
+ |  BIOS data area       0040:0000 00400  0100 (256)  @1024
+ |4 bootopts             0050:0000 00500  0100 (256)  @1280 DEF_OPTSEG loaded by minix.o
+ |8 3rd setup data       0060:0000 00600  0200 (512)  @1536 REL_INITSEG (copied by setup)
+ |  ...
+ |  disk track cache     0080:0000 00800  2400 (9216) @2K   DMASEG
+ |  ...
+ |3 1st kblob load addr  0100:0000 01000  2F00 (188K) @4K   DEF_INIT/LOADSEG/ELKS_INITSEG
+ |5 1st setup exec addr  0120:0000 01200  0200 (512)  @4.5K DEF_INIT+20
+ |9 2nd kernel load&exec 02D0:0000 2D000  2F00 (188K) @11K  REL_SYSSEG (relocated by setup)
+ |1 1st boot exec addr   07C0:0000 07C00  0200 (512)  @31K  BOOTADDR (loaded by BIOS at boot)
+ |                       0800:0000 08000  0000 (32K)  @32K
+ |                       1000:0000 10000  0000        @64K
+ |6 1st kernel load addr 1300:0000 13000  2F00 (188K) @76K  DEF_SYSSEG (copied by setup)
+ |  ...
+ |  ...
+ |7 2nd setup exec addr  9000:0000 90000 00200 (512)  @576K copied by setup
+ |2 2nd boot exec addr   9000:0000 90000 00200 (512)  @576K ES=DS=SS=CS (tiny model) copied
+ !  bios_sect disk buf   9000:0200 90200 00200 (512)  @576.5K (FAT BPB or minix payload here)
+ |  top of 640k ram      A000:0000 A0000 A0000        @640K
+ +-------------------------------------------------------------------------------------------+
+
+1	BIOS loads disk sector 1 (bios_sect.S) to 07C0:0000
+2	bios_sect.S copies itself to high memory 9000:0000 (8000:0000 on QEMU)
+	bios_sect stack grows down from 9000:0000
+
+	MINIX bios_sect calls payload at 9000:0200 (minix_boot.S, tiny model CS=DS=SS)
+	bios_sect .text and .data is ~03F6, linked with minix_boot.o is 1K boot sector
+	payload .data (offset after .text and .rodata) starts around 9000:03F6 to 9000:1406
+3	payload reads MINIX superblock/fs and loads /linux as blob to 0100:0000 (DEF_INIT)
+4	payload reads MINIX superblock/fs and loads /bootopts sector to 0050:0000 (DEF_OPTSEG)
+	boot_sect saves elks_flags (and other?) to 0100:0000 (DEF_INIT, which is SETUP data seg)
+		set by boot_sect: elks_flags
+		set by build: setup_sect, syssize
+5	boot_sect checks ELKS setup signature and jumps to setup code seg 0120:0000 (DEF_INIT+20)
+
+	FAT bios_sect reads FAT BPB within itself at 9000:0000 and loads FAT root dir at 9000:0200
+	boot_sect .text is ~01E2, .data starts at ~01f7, standalone size is 512 byte boot sector
+3	boot_sect reads FAT root dir to load LINUX as blob to 0100:0000 (DEF_INIT)
+(4)		no room to load BOOTOPTS sector to 0050:0000 (DEF_OPTSEG)
+	boot_sect saves elks_flags (and other?) to 0100:0000 (DEF_INIT, which is SETUP data seg)
+5	boot_sect checks ELKS setup signature and jumps to setup code seg 0120:0000 (DEF_INIT+20)
+	setup uses root dir at 9000:0200 and BPB at 9000:0000 to load BOOTOPTS sector to 0050:0000
+
+6	setup CS at 0120:0000 (DEF_INIT+20) checks ELKS sig and copies kernel to 1300:0 (DEF_SYSSEG)
+6+	FAT only - setup reads high mem root dir 9000:0200 to get BOOTOPTS cluster
+		uses high mem BPB at 9000:0000 for secs_per_cluster, etc
+7	setup copies its code segment from 0120:0000 (DEF_INIT+2) to high memory 9000:0 (8000 qemu)
+8	setup now at 9000:0000 copies its data seg from 0100:0 (DEF_INIT) to 0700:0 (REL_INITSEG)
+9	setup copies&relocates kernel from 1300:0000 (DEF_SYSSEG) to 02D0:0000 (REL_SYSSEG)
+	DMASEG starts after 0700:0000 (REL_INITSEG) at 0900:0000 to 02D0:0000 (REL_SYSSEG)
+9+	finally, setup jumps to kernel at 02D0:0000 (REL_SYSSEG)
+
+Current kernel is 80K in size (=14000h)
+Boot loader:
+	0100:0000 DEF_INIT @4K
+		+ 1400:0000 size of kernel (80K)
+	1500:0000 end of 80K kernel
+	3000:0000 max end of 188K kernel @192K
+Setup.S:
+	1300:0000 DEF_SYSSEG @76K
+		+ 1400:0000 size of kernel (80K)
+	2700:0000 end of 80K kernel
+	4200:0000 max end of 188K kernel @264K

bootblocks/boot_sect.S

@@ -36,7 +36,6 @@
 #endif
 
 #define LOADSEG       DEF_INITSEG
-#define ELKS_INITSEG  DEF_INITSEG
 
 // Whether to try to do whole-track reads, or read one sector at a time
 //
@@ -595,26 +594,19 @@ not_elks:
 	jmp _except
 
 boot_it:
-.if LOADSEG != ELKS_INITSEG
-	.error
-.endif
 
 	// Signify that /linux was loaded as 1 blob
-.if ((EF_AS_BLOB|EF_BIOS_DEV_NUM) & 0xff) == 0
-	orb $(EF_AS_BLOB|EF_BIOS_DEV_NUM)>>8,elks_flags+1
-.else
-	orw $(EF_AS_BLOB|EF_BIOS_DEV_NUM),elks_flags
-.endif
+	orb $(EF_AS_BLOB|EF_BIOS_DEV_NUM),elks_flags
 	mov %dx,root_dev
 	mov %cx,part_offset  // save sector offset of booted partition
 	mov %bx,part_offset+2
 
 	RESTORE_DDPT
 
-	mov $ELKS_INITSEG,%ax
+	mov $LOADSEG,%ax
 	mov %ax,%ds
 	mov %ax,%es
-	ljmp $ELKS_INITSEG+0x20,$0
+	ljmp $LOADSEG+0x20,$0
 #endif
 
 //------------------------------------------------------------------------------

bootblocks/boot_sect_fat.h

@@ -213,7 +213,7 @@
 	shr %cl,%ax
 	add %bx,%ax
 
-	// Load the file as one single blob at ELKS_INITSEG:0
+	// Load the file as one single blob at LOADSEG:0
 	mov 0x1d(%si),%dx		// File size divided by 0x100
 #if defined(CONFIG_IMG_FD1232)
 	shr %dx
@@ -225,17 +225,17 @@
 	// #issuecomment-581034966), BIOS reads may reportedly fail if %es
 	// for read calls is not aligned to a large power of 2; to be on the
 	// safe side, rewrite 0x100:0 into something like 0:0x1000...)
-	mov $ELKS_INITSEG,%cx
+	mov $LOADSEG,%cx
 	mov %cx,%es
-.if (ELKS_INITSEG & 0xff) == 0
-	mov $ELKS_INITSEG>>4,%ch
+.if (LOADSEG & 0xff) == 0
+	mov $LOADSEG>>4,%ch
 .else
-	mov $ELKS_INITSEG<<4,%cx
+	mov $LOADSEG<<4,%cx
 .endif
-.if (ELKS_INITSEG & 0xf000) == 0
+.if (LOADSEG & 0xf000) == 0
 	xor %bx,%bx
 .else
-	mov $ELKS_INITSEG&0xf000,%bx
+	mov $LOADSEG&0xf000,%bx
 .endif
 	push %bx
 	call disk_read
@@ -260,15 +260,11 @@
 	push %es
 	pop %ds
 	// Signify that /linux was loaded as 1 blob
-.if ((EF_AS_BLOB|EF_BIOS_DEV_NUM) & 0xff) == 0
-	orb $(EF_AS_BLOB|EF_BIOS_DEV_NUM)>>8,elks_flags+1
-.else
-	orw $(EF_AS_BLOB|EF_BIOS_DEV_NUM),elks_flags
-.endif
+	orb $(EF_AS_BLOB|EF_BIOS_DEV_NUM),elks_flags
 	mov %ax,root_dev
 	mov %cx,part_offset  // save sector offset of booted partition
 	mov %si,part_offset+2
-	ljmp $ELKS_INITSEG+0x20,$0
+	ljmp $LOADSEG+0x20,$0
 
 kernel_name:
 	.ascii "LINUX      "

elks/arch/i86/boot/bootsect.S

@@ -565,7 +565,8 @@ root_flags:
 syssize:
 	.word SYSSIZE
 elks_flags:
-	.word ELKSFLAGS
+	.byte EF_NONE
+	.byte 0
 ram_size:
 	.word RAMDISK
 vid_mode:

elks/arch/i86/boot/setup.S

@@ -70,7 +70,8 @@
 !	0x1f1:	setup_sects	byte in 512-byte sectors, written by build tool
 !	0x1f2:	ROOTFLAGS	word UNUSED
 !	0x1f4:	syssize		word in paragraphs, written by build tool
-!	0x1f6:	elks_flags	word EF_xxx
+!	0x1f6:	elks_flags	byte EF_xxx
+!	0x1f7:				byte UNUSED
 !	0x1f8:	RAMDISK		word UNUSED
 !	0x1fa:	SVGA_MODE	word UNUSED
 !	0x1fc:	root_dev	word Either BIOS boot device or actual kdev_t ROOT_DEV
@@ -152,11 +153,7 @@ no_sig_mess:	.ascii	"No ELKS setup signature found ..."
 chk_blob:
 	mov	$INITSEG,%ax
 	mov	%ax,%ds
-.if (EF_AS_BLOB & 0xff) == 0
-	testb	$(EF_AS_BLOB>>8),elks_flags+1
-.else
-	testw	$EF_AS_BLOB,elks_flags
-.endif
+	testb	$EF_AS_BLOB,elks_flags
 	jz	no_blob
 	std			// move backwards --- we are moving from a
 				// lower address to a higher one

elks/include/linuxmt/boot.h

@@ -17,13 +17,12 @@
 #endif
 
 /* ELKS flags */
-
 #define EF_NONE		0
-#define EF_AS_BLOB	0x8000		/* says that setup and kernel are
+#define EF_AS_BLOB	0x01		/* says that setup and kernel are
 					   loaded as one blob at SETUPSEG:0,
 					   and setup may need to move kernel
 					   to SYSSEG:0 */
-#define EF_BIOS_DEV_NUM	0x4000		/* says that root_dev does not give
+#define EF_BIOS_DEV_NUM	0x02		/* says that root_dev does not give
 					   a <major, minor> block device
 					   number, but only a BIOS drive
 					   number (and possibly other
@@ -32,8 +31,6 @@
 					   figure out the correct root
 					   device */
 
-#define ELKSFLAGS	EF_NONE
-
 /* If we are not building the (dummy) boot sector (elks/elks/arch/i86/boot/
    {bootsect.S, netbootsect.S}) at the start of /linux, then define
    constants giving the offsets of various fields within /linux.
@@ -53,7 +50,7 @@
 #define elks_magic	0x1e6		/* long "ELKS" (45 4c 4b 53) checked by bootsect.S*/
 #define setup_sects	0x1f1		/* byte 512-byte sectors used by setup.S*/
 #define syssize		0x1f4		/* word paragraph kernel size used by setup.S*/
-#define elks_flags	0x1f6		/* word 16-bit ELKS flags, BLOB and BIOS_DRV*/
+#define elks_flags	0x1f6		/* byte ELKS flags, BLOB and BIOS_DRV*/
 #define root_dev	0x1fc		/* word BIOS drive or kdev_t ROOT_DEV*/
 #define boot_flag	0x1fe		/* word constant AA55h*/
 #endif

elks/include/linuxmt/config.h

@@ -28,7 +28,7 @@
 #define SETUP_CPU_TYPE		setupb(0x20)	/* processor type */
 #define SETUP_MEM_KBYTES	setupw(0x2a)	/* base memory in 1K bytes */
 #define SETUP_ROOT_DEV		setupw(0x1fc)	/* root device, kdev_t or BIOS dev */
-#define SETUP_ELKS_FLAGS	setupw(0x1f6)	/* flags for root device type */
+#define SETUP_ELKS_FLAGS	setupb(0x1f6)	/* flags for root device type */
 #define SETUP_PART_OFFSETLO	setupw(0x1e2)	/* partition offset low word */
 #define SETUP_PART_OFFSETHI	setupw(0x1e4)	/* partition offset high word */
 #ifdef CONFIG_ROMCODE
@@ -43,7 +43,7 @@
 #define SETUP_CPU_TYPE		1	/* processor type = 8086 */
 #define SETUP_MEM_KBYTES	640	/* base memory in 1K bytes */
 #define SETUP_ROOT_DEV		setupw(0x1fc)	/* root device, kdev_t or BIOS dev */
-#define SETUP_ELKS_FLAGS	setupw(0x1f6)	/* flags for root device type */
+#define SETUP_ELKS_FLAGS	setupb(0x1f6)	/* flags for root device type */
 #define SETUP_PART_OFFSETLO	setupw(0x1e2)	/* partition offset low word */
 #define SETUP_PART_OFFSETHI	setupw(0x1e4)	/* partition offset high word */
 #define SYS_CAPS		0	/* no XT/AT capabilities */