second stage loader loading semi-works

[bootcensus] / src / boot / boot.s

diff --git a/src/boot/boot.s b/src/boot/boot.s
index ffd1408..ac4a424 100644 (file)
--- a/src/boot/boot.s
+++ b/src/boot/boot.s
@@ -3,15 +3,17 @@
 
        cli
        cld
-       # move stack to the top of 512k
-       mov $0x7000, %ax
+       # move stack to just below the code
+       xor %ax, %ax
        mov %ax, %ss
-       xor %sp, %sp
+       mov $0x7c00, %sp
        # use the code segment for data access
        mov %cs, %ax
        mov %ax, %ds
        mov %ax, %es
 
+       mov %dl, drive_number
+
        call clearscr
 
        mov $_boot2_size, %eax
@@ -23,7 +25,7 @@
        shr $9, %eax
        and $0x1ff, %ebx
        jz 0f
-       inc %eax
+       inc %ax
 0:     pushw %ax
        pushw $1
        # set es to the start of the destination buffer to allow reading in
@@ -32,7 +34,7 @@
        shr $4, %bx
        mov %bx, %es
        xor %bx, %bx
-       call readsect
+       call read_sectors
        jmp boot2_addr
 
        cli
@@ -42,38 +44,104 @@
 
        .set ARG_NSECT, 6
        .set ARG_SIDX, 4
-       .set VAR_NTRACKS, -2
 
-# readsect(first, num)
-readsect:
+# read_sectors(first, num)
+read_sectors:
+       push %bp
+       mov %sp, %bp
+
+       mov ARG_SIDX(%bp), %ax
+       xor %cx, %cx
+
+       jmp 1f
+0:     push %ax
+       call read_sector
+       pop %ax
+       inc %ax
+       inc %cx
+1:     cmp ARG_NSECT(%bp), %cx
+       jnz 0b
+
+       pop %bp
+       ret
+
+       .set VAR_ATTEMPTS, -2
+
+# read_sector(sidx)
+read_sector:
        push %bp
        mov %sp, %bp
        sub $2, %sp
+       push %cx
+       push %dx
+
+       movw $3, VAR_ATTEMPTS(%bp)
 
-       # calculate how many tracks to read
-       mov ARG_NSECT(%bp), %ax
+.Lread_try:
+       # calculate the track (sidx / sectors_per_track)
+       mov 4(%bp), %ax
        xor %dx, %dx
        mov $SECT_PER_TRACK, %cx
        div %cx
-       cmp $0, %dx
-       jz 0f
-       inc %ax
-0:     mov %ax, VAR_NTRACKS(%bp)
+       mov %ax, %cx
+       # save the remainder in ax
+       mov %dx, %ax
+       # head in dh
+       mov %cl, %dh
+       and $1, %dh
+       # cylinder (track/2) in ch [0-7] and cl[6,7]<-[8,9]
+       rol $7, %cx
+       ror $2, %cl
+       and $0xc0, %cl
+       # sector num cl[0-5] is sidx % sectors_per_track (saved in ax)
+       inc %al
+       or %al, %cl 
+       # ah = 2 (read), al = 1 sectors
+       mov $0x0201, %ax
+       movb drive_number, %dl
+       int $0x13
+       jnc .Lread_ok
+
+       # abort after 3 attempts
+       decw VAR_ATTEMPTS(%bp)
+       jz .Lread_fail
+
+       # error detected, reset controller and retry
+       xor %ah, %ah
+       int $0x13
+       jmp .Lread_try
+
+.Lread_fail:
+       mov 4(%bp), %ax
+       jmp abort_read
+
+.Lread_ok:
+       # increment es:bx accordingly (advance es if bx overflows)
+       add $512, %bx
+       jno 0f
+       mov %es, %ax
+       add $4096, %ax
+       mov %ax, %es
 
-       xor %cx, %cx
-0:     cmp VAR_NTRACKS(%bp), %cx
-       jz 0f
-       push %cx
-       call read_track
+0:     pop %dx
        pop %cx
-       jmp 0b
-0:
-       # TODO cont.
+       add $2, %sp
        pop %bp
        ret
 
-read_track:
-       ret
+str_read_error: .asciz "Failed to read sector: "
+
+abort_read:
+       push %ax
+       mov $str_read_error, %si
+       call print_str
+
+       xor %eax, %eax
+       pop %ax
+       call print_num
+
+       cli
+       hlt
 
 clearscr:
        push %es
@@ -86,6 +154,29 @@ clearscr:
        pop %es
        ret
 
+cursor_x: .byte 0
+
+# expects string pointer in ds:si
+print_str:
+       push %es
+       pushw $0xb800
+       pop %es
+       xor %di, %di
+       movb $0, cursor_x
+
+0:     mov (%si), %al
+       mov %al, %es:(%di)
+       movb $7, %es:1(%di)
+       inc %si
+       add $2, %di
+       incb cursor_x
+       cmp $0, %al
+       jnz 0b
+
+       pop %es
+       ret
+
+# expects number in eax
 print_num:
        # save es
        push %es
@@ -106,7 +197,10 @@ print_num:
        # print the backwards string
        pushw $0xb800
        pop %es
-       xor %di, %di
+       movb cursor_x, %al
+       xor %ah, %ah
+       shl $1, %ax
+       mov %ax, %di
 
 0:     dec %si
        mov (%si), %al
@@ -122,6 +216,7 @@ print_num:
        pop %es
        ret
 
+drive_number: .byte 0
 numbuf: .space 10
        .org 510
        .byte 0x55