FDOS · PerditionC · Feb 18, 2024 · Feb 6, 2024 · Feb 6, 2024 · Feb 6, 2024
diff --git a/boot/boot.asm b/boot/boot.asm
@@ -24,38 +24,59 @@
 ; License along with DOS-C; see the file COPYING.  If not,
 ; write to the Free Software Foundation, 675 Mass Ave,
 ; Cambridge, MA 02139, USA.
-;
-;
-;       +--------+ 1FE0:7E00
-;       |BOOT SEC|
-;       |RELOCATE|
-;       |--------| 1FE0:7C00
-;       |LBA PKT |
-;       |--------| 1FE0:7BC0
-;       |--------| 1FE0:7BA0
-;       |BS STACK|
-;       |--------|
-;       |4KBRDBUF| used to avoid crossing 64KB DMA boundary
-;       |--------| 1FE0:63A0
-;       |        |
-;       |--------| 1FE0:3000
-;       | CLUSTER|
-;       |  LIST  |
-;       |--------| 1FE0:2000
-;       |        |
-;       |--------| 0000:7E00
-;       |BOOT SEC| overwritten by max 128k FAT buffer
-;       |ORIGIN  | and later by max 134k loaded kernel
-;       |--------| 0000:7C00
-;       |        |
-;       |--------|
-;       |KERNEL  | also used as max 128k FAT buffer
-;       |LOADED  | before kernel loading starts
-;       |--------| 0060:0000
-;       |        |
-;       +--------+
 
 
+; Memory layout for the FreeDOS FAT12/FAT16 boot process:
+;
+;	...
+;	|-------| 1FE0h:7E00h = 27C00h (159 KiB)
+;	|BOOTSEC| loader relocates itself here first thing,
+;	|RELOC.	|  before loading root directory/FAT/kernel file
+;	|-------| 1FE0h:7C00h = 27A00h (158 KiB)
+;	|  gap  | PARAMS live here
+;	|LBA PKT| LBA disk packet
+;	|-------| 1FE0h:7BC0h = 279C0h (158 KiB)
+;	|  gap  | READADDR_* live here
+;	|-------| 1FE0h:7BA0h = 279A0h (158 KiB)
+;	| STACK | below relocated loader, above sector buffer (size 5.9 KiB)
+;	...
+;	|-------| 1FE0h:6400h = 26200h (152 KiB)
+;	|SEC.BUF| sector buffer, to avoid crossing 64 KiB DMA boundary (size 8 KiB)
+;	|-------| 1FE0h:4400h = 24200h (144 KiB)
+;	...
+;	|-------| 1FE0h:4380h = 24182h (144 KiB)
+;	|CLUSTER| built from FAT, listing every cluster of the kernel file.
+;	| LIST  |  file <= 134 KiB, cluster >= 32 Byte, hence <= 8578 B list.
+;	|-------| 1FE0h:2200h = 22000h (136 KiB)
+;	...
+;	|-------| 0000h:7E00h = 07E00h (31.5 KiB)
+;	|BOOTSEC| possibly overwritten by the FAT (<= 128 KiB) and the kernel,
+;	|ORIGIN |  so the bootsector relocates itself up...
+;	|-------| 0000h:7C00h = 07C00h (31 KiB)
+;	...
+;	|-------|
+;	|KERNEL	| maximum size 128 KiB (overwrites bootsec origin)
+;	|LOADED	| (holds directory then FAT before kernel file load)
+;	|-------| 0060h:0000h = 00600h (1.5 KiB)
+;	...
+; The entire root directory is loaded to the kernel load address
+;  to scan for the kernel file. It is assumed to fit into 128 KiB.
+;  Typical root directory size is up to 512 entries = 16 KiB.
+;  Further, it is assumed that at least one root directory entry
+;  starts with a NUL byte to signify the end of the directory.
+; After the directory entry is found, the entire FAT is loaded to
+;  the kernel load address. It is assumed that the size of the FAT
+;  in sectPerFat will not lead to a FAT larger than 128 KiB, which
+;  is the maximum size a FAT16 may fully utilise.
+; The kernel load segment may be patched using the SYS /L switch.
+;  We support values between 0x60 and 0x200 here, with file size
+;  of up to 128 KiB (rounded to cluster size). Default is 0x60.
+; This loader traditionally supports file sizes up to 134 KiB,
+;  assuming the default segment of 0x60. This does require a
+;  cluster size of 4 KiB (leads to maximum 132 KiB) or 2 KiB or
+;  lower (maximum 134 KiB). A more portable maximum is 128 KiB,
+;  which works with cluster sizes up to 128 KiB.
+
 ;%define ISFAT12         1
 ;%define ISFAT16         1
 ;verify one and only one of ISFAT12 or ISFAT16 is defined
@@ -67,6 +88,10 @@
   %error Must select one FS
 %endif
 
+                ; NOTE: sys must be updated if magic offsets change
+%assign ISFAT1216DUAL 1
+	%include "magic.mac"
+
 
 segment .text
 
@@ -99,8 +124,8 @@ Entry:          jmp     short real_start
 
 %define LOADSEG         0x0060
 
-%define FATBUF          0x2000          ; offset of temporary buffer for FAT
-                                        ; chain
+%define CLUSTLIST       0x2200          ; offset of temporary buffer for FAT
+                                        ; chain cluster list
 
 ;       Some extra variables
 
@@ -134,9 +159,9 @@ Entry:          jmp     short real_start
 %define LBA_SECTOR_32  word [LBA_PACKET+12]
 %define LBA_SECTOR_48  word [LBA_PACKET+14]
 
-%define READBUF 0x63A0 ; max 4KB buffer (min 2KB stack), == stacktop-0x1800
-%define READADDR_OFF   BP-0x60-0x1804    ; pointer within user buffer
-%define READADDR_SEG   BP-0x60-0x1802
+%define READBUF 0x4400                      ; max 8 KiB buffer
+%define READADDR_OFF   word BP-0x60         ; pointer within user buffer
+%define READADDR_SEG   word BP-0x60+2
 
 %define PARAMS LBA_PACKET+0x10
 ;%define RootDirSecs     PARAMS+0x0         ; # of sectors root dir uses
@@ -178,6 +203,7 @@ real_start:
                 jmp     word 0x1FE0:cont
 
 loadseg_off     dw      0
+	magicoffset "loadseg", 5Ch, 5Ch
 loadseg_seg     dw      LOADSEG
 
 cont:
@@ -190,6 +216,7 @@ cont:
 ; in DL, however we work around this in SYS.COM by NOP'ing out the use of DL
 ; (formerly we checked for [drive]==0xff; update sys.c if code moves)
 ;
+	magicoffset "set unit", 66h, 66h
                 mov     [drive], dl     ; rely on BIOS drive number in DL
 
                 mov     LBA_SIZE, 10h
@@ -294,7 +321,7 @@ ffDone:
                 push    ds
                 pop     es
                 mov     ds, [loadseg_60]
-                mov     di, FATBUF
+                mov     di, CLUSTLIST
 
 next_clust:     stosw                           ; store cluster number
                 mov     si, ax                  ; SI = cluster number
@@ -351,7 +378,7 @@ finished:       ; Mark end of FAT chain with 0, so we have a single
 
                 les     bx, [loadsegoff_60]   ; set ES:BX to load address 60:0
 
-                mov     si, FATBUF      ; set DS:SI to the FAT chain
+                mov     si, CLUSTLIST           ; set DS:SI to the FAT chain
 
 cluster_next:   lodsw                           ; AX = next cluster to read
                 or      ax, ax                  ; EOF?
@@ -426,20 +453,7 @@ read_next:
                 mov     bx,055aah               ;
                 mov     dl, [drive]
 
-                ; NOTE: sys must be updated if location changes!!!
-%ifdef ISFAT12
-  %define LBA_TEST_OFFSET 17Bh
-%elifdef ISFAT16
-  %define LBA_TEST_OFFSET 178h
-%else
-  %define LBA_TEST_OFFSET 0
-                ; Just a placeholder, so the proper error message
-                ;  will be shown when assembling without either
-                ;  of the ISFATx defines.
-%endif
-%if ($ - Entry) != LBA_TEST_OFFSET
-    %error Must update constant offset (LBA_TEST_OFFSET) to test dl,dl here and in sys.c for FATFS
-%endif
+	magicoffset "LBA detection", 17Bh, 178h
                 test    dl,dl                   ; don't use LBA addressing on A:
                 jz      read_normal_BIOS        ; might be a (buggy)
                                                 ; CDROM-BOOT floppy emulation
@@ -543,6 +557,7 @@ do_int13_read:
 
        times   0x01f1-$+$$ db 0
 
+	magicoffset "kernel name", 1F1h, 1F1h
 filename        db      "KERNEL  SYS",0,0
 
 sign            dw      0xAA55

diff --git a/boot/boot32.asm b/boot/boot32.asm
@@ -1,34 +1,37 @@
-;	+--------+
-;	|        |
-;	|        |
-;	|--------| 4000:0000
-;	|        |
-;	|  FAT   |
-;	|        |
-;	|--------| 2000:0000
-;	|BOOT SEC|
-;	|RELOCATE|
-;	|--------| 1FE0:0000
-;	|        |
-;	|        |
-;	|        |
-;	|        |
-;	|--------|
-;	|BOOT SEC|
-;	|ORIGIN  | 07C0:0000
-;	|--------|
-;	|        |
-;	|        |
-;	|        |
-;	|--------|
-;	|KERNEL  |
-;	|LOADED  |
-;	|--------| 0060:0000
-;	|        |
-;	+--------+
+
+; Memory layout for the FreeDOS FAT32 single stage boot process:
+;
+;	...
+;	|-------| 1FE0h:7E00h = 27C00h (159 KiB)
+;	|BOOTSEC| loader relocates itself here first thing,
+;	|RELOC.	|  before loading root directory/FAT/kernel file
+;	|-------| 1FE0h:7C00h = 27A00h (158 KiB)
+;	| STACK | below relocated loader, above FAT sector (size 22 KiB)
+;	...
+;	|-------| 2200h:2000h = 24000h (144 KiB)
+;	|  FAT  | (only 1 sector buffered, maximum sector size 8 KiB)
+;	|-------| 2200h:0000h = 22000h (136 KiB)
+;	...
+;	|-------| 0000h:7E00h = 07E00h (31.5 KiB)
+;	|BOOTSEC| overwritten by the kernel, so the
+;	|ORIGIN | bootsector relocates itself up...
+;	|-------| 0000h:7C00h = 07C00h (31 KiB)
+;	...
+;	|-------|
+;	|KERNEL	| maximum size 128 KiB (overwrites bootsec origin)
+;	|LOADED	| (holds 1 sector directory buffer before kernel file load)
+;	|-------| 0060h:0000h = 00600h (1.5 KiB)
+;	...
+; The kernel load segment may be patched using the SYS /L switch.
+;  We support values between 0x60 and 0x200 here, with file size
+;  of up to 128 KiB (rounded to cluster size). Default is 0x60.
 
 ;%define MULTI_SEC_READ  1
 
+                ; NOTE: sys must be updated if magic offsets change
+%assign ISFAT1216DUAL 0
+	%include "magic.mac"
+
 
 segment	.text
 
@@ -66,7 +69,7 @@ Entry:          jmp     short real_start
 
 %define LOADSEG         0x0060
 
-%define FATSEG          0x2000
+%define FATSEG          0x2200
 
 %define fat_sector      bp+0x48         ; last accessed sector of the FAT
 
@@ -97,12 +100,14 @@ real_start:     cld
 		jmp     word 0x1FE0:cont
 
 loadseg_off	dw	0
+	magicoffset "loadseg", 78h
 loadseg_seg	dw	LOADSEG
 
 cont:           mov     ds, ax
                 mov     ss, ax
                 lea     sp, [bp-0x20]
 		sti
+	magicoffset "set unit", 82h
                 mov     [drive], dl     ; BIOS passes drive number in DL
 
 ;                call    print
@@ -402,6 +407,7 @@ no_incr_es:
 
        times   0x01f1-$+$$ db 0
 
+	magicoffset "kernel name", 1F1h
 filename        db      "KERNEL  SYS",0,0
 
 sign            dw      0xAA55
diff --git a/boot/boot32lb.asm b/boot/boot32lb.asm
@@ -27,27 +27,36 @@
 
 
 ; Memory layout for the FreeDOS FAT32 single stage boot process:
-
+;
 ;	...
-;	|-------| 1FE0:7E00
-;	|BOOTSEC|
-;	|RELOC.	|
-;	|-------| 1FE0:7C00
+;	|-------| 1FE0h:7E00h = 27C00h (159 KiB)
+;	|BOOTSEC| loader relocates itself here first thing,
+;	|RELOC.	|  before loading root directory/FAT/kernel file
+;	|-------| 1FE0h:7C00h = 27A00h (158 KiB)
+;	| STACK | below relocated loader, above FAT sector (size 22 KiB)
 ;	...
-;	|-------| 2000:0200
-;	|  FAT  | (only 1 sector buffered)
-;	|-------| 2000:0000
+;	|-------| 2200h:2000h = 24000h (144 KiB)
+;	|  FAT  | (only 1 sector buffered, maximum sector size 8 KiB)
+;	|-------| 2200h:0000h = 22000h (136 KiB)
 ;	...
-;	|-------| 0000:7E00
+;	|-------| 0000h:7E00h = 07E00h (31.5 KiB)
 ;	|BOOTSEC| overwritten by the kernel, so the
 ;	|ORIGIN | bootsector relocates itself up...
-;	|-------| 0000:7C00
+;	|-------| 0000h:7C00h = 07C00h (31 KiB)
 ;	...
 ;	|-------|
-;	|KERNEL	| maximum size 134k (overwrites bootsec origin)
-;	|LOADED	| (holds 1 sector directory buffer before kernel load)
-;	|-------| 0060:0000
+;	|KERNEL	| maximum size 128 KiB (overwrites bootsec origin)
+;	|LOADED	| (holds 1 sector directory buffer before kernel file load)
+;	|-------| 0060h:0000h = 00600h (1.5 KiB)
 ;	...
+; The kernel load segment may be patched using the SYS /L switch.
+;  We support values between 0x60 and 0x200 here, with file size
+;  of up to 128 KiB (rounded to cluster size). Default is 0x60.
+
+                ; NOTE: sys must be updated if magic offsets change
+%assign ISFAT1216DUAL 0
+	%include "magic.mac"
+
 
 segment	.text
 
@@ -85,7 +94,7 @@ Entry:		jmp	short real_start
 
 %define LOADSEG		0x0060
 
-%define FATSEG		0x2000
+%define FATSEG		0x2200
 
 %define	fat_secshift	fat_afterss-1	; each fat sector describes 2^??
 					; clusters (db) (selfmodifying)
@@ -125,14 +134,17 @@ real_start:	cld
 		rep	movsw		; move boot code to the 0x1FE0:0x0000
 		jmp	word 0x1FE0:cont
 
-loadseg_off	dw	0, LOADSEG
+loadseg_off	dw	0
+	magicoffset "loadseg", 78h
+		dw	LOADSEG
 
 ; -------------
 
 cont:		mov	ds, ax
 		mov	ss, ax		; stack and BP-relative moves up, too
                 lea     sp, [bp-0x20]
 		sti
+	magicoffset "set unit", 82h
 		mov	[drive], dl	; BIOS passes drive number in DL
 
 %ifndef QUIET
@@ -409,6 +421,7 @@ msg_BootError	db "No "
 		; currently, only "kernel.sys not found" gives a message,
 		; but read errors in data or root or fat sectors do not.
 
+	magicoffset "kernel name", 1F1h
 filename	db "KERNEL  SYS"
 
 sign		dw 0, 0xAA55