Fixes and improvements to FAT32 driver

* Fix issue where multiple clusters would be loaded 0x800 apart rather than
  contiguously.
* Get rid of some global variables, saving space.
* si points to the start LBA address rather than the partition table entry,
  saving a bit of space as we no longer need to use offsets.
* In read_cluster_chain, drop push/popad as they in fact do not
  save the upper 16 bytes of the registers. Instead, just push
  and pops the registers that are used.

diff --git a/boot/x86/fat32-structs.s b/boot/x86/fat32-structs.s
index 23980ba426c2d7a16c24383cb2622a0b0c03e380..e72735d82bd29a29a7c80224a50c7a1134d9bed0 100644 (file)
--- a/boot/x86/fat32-structs.s
+++ b/boot/x86/fat32-structs.s
@@ -1,15 +1,6 @@
 ; FAT32 data structures
 ; Because they have to be defined before use
 
-struc part_entry
-       .attrib: resb 1
-       .chs_start: resb 3
-       .type: resb 1
-       .chs_end: resb 3
-       .lba_start: resd 1
-       .lba_end: resd 1
-endstruc
-
 struc dir_entry
        .name: resb 11
        .attr: resb 1

diff --git a/boot/x86/fat32.s b/boot/x86/fat32.s
index 31288cb92cad9433592f3a2a3e090895d26b187d..caed0361a311af54737594d17b47ae644f517707 100644 (file)
--- a/boot/x86/fat32.s
+++ b/boot/x86/fat32.s
@@ -1,12 +1,20 @@
 ; FAT32 driver
-; BOOT_DRIVE, part_table_entry and bp must be set up by the code using this driver.
-; first_data_sec is to be set up by calling get_1st_data_sec.
+; BOOT_DRIVE, si and bp must be set up by the code using this driver.
+; (Specifically, si must be a pointer to the partition's partition
+; table entry and bp must be a pointer to the BPB.)
+; Before reading any cluster chains, callers must call get_1st_data_sec
+; and must be careful not to trash ecx after calling it.
 
 ; eax - start cluster number
 ; es:di - where to load the cluster chain
 ; NOTE: Cluster chain might be the root directory or a file.
 read_cluster_chain:
-       pusha
+       push eax
+       push ebx
+       push ecx
+       push edx
+       push es
+       push di
 .loop:
        ; get the first sector of the cluster to read
        push eax
@@ -14,13 +22,15 @@ read_cluster_chain:
        xor ebx, ebx
        mov bl, BPB_SecPerClus
        mul ebx
-       add eax, [first_data_sec]
+       add eax, ecx
        mov ebx, eax
        pop eax
 
        ; read the cluster
+       push cx
        movzx cx, BPB_SecPerClus
        call read_sectors
+       pop cx
 
        ; increment the load offset (and segment if needed)
        push eax
@@ -36,22 +46,21 @@ read_cluster_chain:
 .get_next_cluster:
        pop eax
        ; get FAT sector number and offset containing the next cluster number
-       xor edx, edx
-       mov ebx, 4
-       mul dword ebx
+       xor ebx, ebx
+       mov bl, 4 ; put 4 in EBX, doing this instead of "mov ebx, 4" saves us an entire byte
+       mul ebx
        movzx ebx, word BPB_BytsPerSec
-       div dword ebx
+       div ebx
        movzx ebx, word BPB_RsvdSecCnt
        add eax, ebx
-       push bp
-       mov bp, [part_table_entry]
-       add eax, [bp+part_entry.lba_start]
-       pop bp
+       add eax, [si]
        mov ebx, edx
        ; reminder for myself: EAX is FAT sector number, (E)BX is offset into FAT sector
        
        ; load the FAT sector we're looking for
 
+       push cx
+       push di
        push ebx ; offset
        push es ; we want to read at 0:1000, not STAGE3_SEGMENT:1000
        push eax ; desired LBA
@@ -65,15 +74,20 @@ read_cluster_chain:
        ; find the cluster we're looking for
        pop es ; restore STAGE3_SEGMENT
        pop ebx ; pop FAT offset back into EBX for cmp
-       cmp dword [di+bx], 0xffffff7
-       jge .done ; if cluster number is greater than or equal to the defective cluster value, we're done
-                 ; TODO: should this perhaps be changed so that equal makes it error out?
-       ; otherwise, load the next sector number in eax and read again
        mov eax, [di+bx]
-       jmp .loop
+       pop di
+       pop cx
+       cmp eax, 0xffffff7
+       jl .loop ; if cluster number is lower than the defective cluster value, we're not done
+                ; TODO: perhaps check is it equal defective and error out in that case?
 .done:
        ; cleanup and return
-       popa
+       pop di
+       pop es
+       pop edx
+       pop ecx
+       pop ebx
+       pop eax
        ret
 
 ; es:di - where to load the sector(s)
@@ -101,18 +115,17 @@ read_sectors:
                hlt
                jmp $-1
 
-; no arguments
+; ds:si - pointer to partition table entry
+; RETURN: ecx - first data sector
 get_1st_data_sec:
        push eax
        push ebx
-       xor eax, eax
        movzx ax, BPB_NumFats 
        mul dword BPB_FatSz32 ; space occupied by all FATs
        movzx ebx, word BPB_RsvdSecCnt
        add eax, ebx ; space occupied by the reserved area
-       mov bx, [part_table_entry]
-       add eax, [bx+part_entry.lba_start] ; space before the partition
-       mov [first_data_sec], eax
+       add eax, [si] ; space before the partition
+       mov ecx, eax
        pop ebx
        pop eax
        ret
@@ -120,5 +133,3 @@ get_1st_data_sec:
 read_error: db "Read error", 0
 
 BOOT_DRIVE: db 0
-part_table_entry: dw 0
-first_data_sec: dd 0

diff --git a/boot/x86/mbr.s b/boot/x86/mbr.s
index 8089b2f27e244bbbe1aeecb1477f691d77152b77..613b24894d993a26fac2891e3ecad4768028cb7d 100644 (file)
--- a/boot/x86/mbr.s
+++ b/boot/x86/mbr.s
@@ -5,7 +5,7 @@ cpu 686
 org 0x600
 
 _start:
-       cli
+       cli ; we really don't want to be interrupted during relocation
        ; set up segment registers
        xor ax, ax
        mov ds, ax
@@ -27,55 +27,51 @@ real_start:
        call check_int13_ext
        mov byte [BOOT_DRIVE], dl
        ; look for active partition
-       mov bx, part_1
+       mov si, part_1
        mov cx, 4
        .check_part_loop:
-               mov al, [bx]
+               mov al, [si]
                test al, 0x80
                jnz .part_found
-               add bx, 16
+               add si, 16
                loop .check_part_loop
                .error:
                        mov si, no_os
                        call print
-                       cli
                        hlt
                        jmp short $-1
        .part_found:
                ; look for any other active partitions, if they exist the partition table is invalid
                cmp cx, 0
                je .load_vbr
-               push bx
+               push si 
        .look_other_loop:
-               add bx, 16
-               mov al, [bx]
+               add si, 16
+               mov al, [si]
                test al, 0x80
                jnz .invalid_mbr
                loop .look_other_loop
        .load_vbr:
                ; load active partition's VBR
-               pop bx
+               pop si
                mov ax, 0x7c0
-               mov edx, [bx+0x8]
-               push bx ; save pointer to partition table entry
+               add si, 8
+               mov edx, [si]
                xor bx, bx
                call read_sector
                ; check is the VBR bootable (ends with 0x55 0xaa), if not halt
                cmp word [0x7dfe], 0xaa55
                jne .not_bootable
                mov dl, [BOOT_DRIVE]
-               pop si ; hand off partition table entry to VBR
-               jmp 0x7c00
+               call 0x7c00
                .not_bootable:
                        mov si, no_os
                        call print
-                       cli
                        hlt
                        jmp short $-1
        .invalid_mbr:
                mov si, invalid_mbr
                call print
-               cli
                hlt
                jmp short $-1
 check_int13_ext:
@@ -92,7 +88,6 @@ check_int13_ext:
        .no_ext:
                mov si, no_int13_ext
                call print
-               cli
                hlt
                jmp short $-1
 
@@ -119,7 +114,6 @@ read_sector:
        .error:
                mov si, read_fail
                call print
-               cli
                hlt
                jmp short $-1
 

diff --git a/boot/x86/vbr-fat32.s b/boot/x86/vbr-fat32.s
index 061ca527afc3d585558c43b495c72a68536c6476..b4c71489e5bf9d421ded07e145550c13e37cb716 100644 (file)
--- a/boot/x86/vbr-fat32.s
+++ b/boot/x86/vbr-fat32.s
@@ -24,7 +24,6 @@ real_start:
 
        ; we expect the boot drive to be in DL and our partition table entry in DS:SI
        mov [BOOT_DRIVE], dl
-       mov [part_table_entry], si
 
        ; calculate the 1st sector of the data area
        call get_1st_data_sec
@@ -37,7 +36,9 @@ real_start:
        mov eax, BPB_RootClus
        call read_cluster_chain
 
+       push cx
        mov cx, 11
+       push si
 .find_stage_3:
        mov si, STAGE3_NAME
        cmp byte [es:di], 0 ; we have no more entries to look at
@@ -53,11 +54,14 @@ real_start:
        add bx, 0x1000
        mov es, bx
 .stage3_found:
-       sub di, 0xb ; "repe cmpsb" incremented this
+       pop si
+       pop cx
+       add di, 9 ; knowing that cmpsb incremented this by 11, we can also increment it by 9
+                      ; right here so we can use 1 less offset below
        ; stage 3 has been found and ES:DI points to its directory entry
-       mov ax, [es:di+dir_entry.firstclushi] ; load high half of the 1st cluster
+       mov ax, [es:di] ; load high half of the 1st cluster
        shl eax, 16
-       mov ax, [es:di+dir_entry.firstcluslo] ; load low half of the 1st cluster
+       mov ax, [es:di+(dir_entry.firstcluslo-dir_entry.firstclushi)] ; load low half of the 1st cluster
        mov bx, STAGE3_SEGMENT
        mov es, bx
        mov di, STAGE3_OFFSET
@@ -70,7 +74,6 @@ real_start:
        mov si, stage3_missing
        call print
 .halt:
-       cli
        hlt
        jmp short $-1