From 6e015763ff15de774e6479bb895cdbcf8479680e Mon Sep 17 00:00:00 2001
From: Andy Kallmeyer <ask@ask.systems>
Date: Fri, 6 Sep 2024 21:47:31 -0700
Subject: [PATCH] More reliably load the extra sectors of code in the
 bootloader

I found out that you're supposed to call the reset BIOS call and retry
several times because real floppy drives act like that. Sure enough when
I checked the BIOS manual it says you should retry!

I thought about using the "actual number of sectors read" return value
to request fewer sectors the next try but there are a few reasons why
not:
  1. I saw online that some BIOS impls don't return a valid value unless
     the status flag is set
  2. It's complicated to calculate the address to write to and
     technically I would have to calculate the track number as well if
     the number of sectors to read is more than 63 (but that's more than
     the size of the Code Segment anyway) and some BIOS support
     multi-track reads from one call so I don't have to calculate that.

So I don't really understand when and how you're supposed to use that
value. I would think you shouldn't trust any data read if you got an
error status.
---
 bootloader/bootsect.asm | 54 ++++++++++++++++++++++++++++++++---------
 1 file changed, 43 insertions(+), 11 deletions(-)

diff --git a/bootloader/bootsect.asm b/bootloader/bootsect.asm
index 964e4de..67a2da2 100644
--- a/bootloader/bootsect.asm
+++ b/bootloader/bootsect.asm
@@ -72,26 +72,34 @@ mov ax, 0x0305
 ;mov bx, 0x0107 ; 500 ms delay before repeat ; 16 characters per second
 mov bx, 0x0100 ; 500 ms delay before repeat ; 30 characters per second
 int 0x16
+; fallthrough
+
+load_extra_sectors:
+; int 0x13 shouldn't be called with 0 for the amount to read.
+; Can't use preprocessor for this unfortunately because it's a label
+mov al, NUM_EXTRA_SECTORS ; number of sectors to read
+test al, al
+je start_
 
 ; Load the code from the extra sectors
+; Try to do the full read every time to take advantage of possible BIOS support
+; for multi-track reads if NUM_EXTRA_SECTORS > 63
 mov ah, 0x02
-mov al, NUM_EXTRA_SECTORS
 mov bx, SECTOR_SIZE ; es:bx is address to write to. es = cs, so write directly after the boot sector
-mov cx, 0x0002 ; Cylinder 0; Sector 2 (1 is the boot sector)
-mov dl, [BOOT_DISK]
-xor dh, dh ; Head 0
+mov dl, [BOOT_DISK] ; Drive number
+xor dh, dh ; Head number
+mov cx, 0x0002 ; Read from Cylinder (track) 0; Sector 2 (1 is the boot sector)
 int 0x13
 
-; Check for errors
-cmp ax, NUM_EXTRA_SECTORS
-je start_
+jnc start_ ; if there was no error, jump to the loaded user code
+; Otherwise handle errors
 
 push ax ; push the error code
 
 ; Print the error message
 mov ax, 0x1301 ; Write String, move cursor mode in al
-mov bp, error_msg ; String pointer in es:bp (es is at code start from bootsect-header.asm)
-mov cx, error_msg_len ; String length
+mov bp, disk_error_msg ; String pointer in es:bp (es is at code start from bootsect-header.asm)
+mov cx, disk_error_msg_len ; String length
 xor dx, dx ; top left
 mov bx, 0x004F ; bh = 0 (page number); bl = color (white on red)
 int 0x10
@@ -99,6 +107,27 @@ int 0x10
 pop cx ; pop the error code
 call print_hex ; print the error code
 
+mov ax, 0x1301 ; Write String, move cursor mode in al
+mov bp, retry_msg ; String pointer in es:bp (es is at code start from bootsect-header.asm)
+mov cx, retry_msg_len ; String length
+mov dx, 0x0100 ; second line; left edge
+mov bx, 0x004F ; bh = 0 (page number); bl = color (white on red)
+int 0x10
+
+mov cl, [READ_RETRIES]
+call print_hex
+
+cmp byte [READ_RETRIES], 0
+je .no_more_tries
+
+; Reset the disk and retry
+dec byte [READ_RETRIES]
+xor ax, ax
+mov dl, [BOOT_DISK] ; Drive number
+int 0x13
+jmp load_extra_sectors
+
+.no_more_tries:
 jmp $ ; stop forever
 
 ; Prints the ascii hex character which represents the integer value of al
@@ -174,9 +203,12 @@ print_hex_byte:
 
 ; === Bootsector data area ===
 
-error_msg: db `Error reading additional sectors from disk: `
-error_msg_len: equ $-error_msg
+disk_error_msg: db `Error reading from disk: `
+disk_error_msg_len: equ $-disk_error_msg
+retry_msg: db `Retry attempts remaining: `
+retry_msg_len: equ $-retry_msg
 
 BOOT_DISK: db 0x00 ; value is filled first thing
+READ_RETRIES: db 5
 
 %include "bootloader/bootsect-footer.asm"