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The code to load the rest of the system uses the BIOS to avoid dealing with the different possible boot media. It either starts loading from where the Master Boot Record indicates (assuming a HardDisk boot), or from the second sector onwards.

It needs to deal with all sorts of intricacies: not the least of which is disk errors! (The code tries a few times before giving up.) But it also has to handle not being able to load all the sectors in one operation: it merely moves the pointers along and tries again.

Memory Map

After this code, the Memory Map will look like this:

Address Usage
0000_0000h Interrupt Vector Table
0000_0400h BIOS Data Area
0000_0500h Available
0000_0600h Master Boot Record (MBR) *
0000_0800h Available
0000_0E70h Loaded Initial IDT values
0000_1000h Loaded Protected Mode code
0000_2070h Available
0000_7C00h SS:SP Stack Top
0000_7C00h BIOS-loaded Boot Sector
0000_7E00h Available
0009_F???h ** Extended BIOS Data Area
000A_0000h Adapter / ROM Area
000B_8000h Text Video Memory
000F_0000h BIOS ROM
0010_0000h High Memory ***

* The MBR only exists here if the PC has booted from a Hard Drive.
** An INT 12h will help identify this value.
*** Note that this memory is only accessible in Protected Mode - except for the first 64 kiB (less 16 bytes), which you can access with a Segment Register set to 0FFFFh as long as the A20 Gate is off. By the way: DON'T DO THIS!


; Boot/
; This module loads the rest of the system - all the Protected Mode stuff.
; It loads it into RAM way down low, leaving the rest of RAM for the system's
; use - and allowing this boot code to be jettisoned.
; The number of Sectors to load. Note that the resultant value may exceed what
; the BIOS can handle: the typical maximum is "number of sectors per track".
; Different floppy formats use different numbers: 8, 9, 15, 18, and 36 are known.
; Since the lowest value is 8, that's the "safest" value. 1.44MB floppies use 18
Load.Sectors    EQU             Load.Size / BIOS.Disk.Sector.Size
; Upon entry, DL is BIOS drive number, ES:SI is Hard Disk Partition entry (if any)
                MOV             AX,Load.Base>>4    ; Segment to load to
;               MOV             ES,AX              ; Not yet! Need BIOS value
                XOR             BX,BX              ; Offset to load to
                TEST            SI,SI              ; Sensible value?
                JZ              .Beginning         ; Not really...
; Assume pointing to MBR Table entry
                MOV             CX,[ES:SI+BIOS.MBR.Entry.StartCylSect]
                MOV             DH,[ES:SI+BIOS.MBR.Entry.StartHead]
; Now check if above assumption is true!
                OR              SI,BIOS.Disk.Sector.Size-1; Round UP to BIOS.Sig
                CMP             [ES:SI-1],WORD BIOS.Sig.Value ; (Well, nearly!)
                JE              .Continue          ; All that hard work panned out!
; Assumption NOT true: no help from BIOS! Just load from first drive sector
                MOV             CX,00_01h          ; Cyl 0, Sect #1
                MOV             DH,0               ; Head Start
                INC             CX                 ; Next sector
;               MOV             DL,                ; DriveNum provided by BIOS
                MOV             SI,3               ; Try this many times
                MOV             ES,AX              ; Can load buffer segment now!
                MOV             AH,BIOS.Disk.Fn.Read ; Read sectors
                MOV             AL,Load.Sectors    ; Number sectors
.NumToRead      EQU             $-1                ; GASP! Self-modifying code!
                INT             BIOS.Disk.Int      ; BIOS Read
                JNC             .ReadSome          ; Carry set on error
                CMP             AL,1               ; Tried single-sector reads?
                JE              .Reset             ; Yep! Better reset drive
                MOV             AL,1               ; No. Try that then.
                JMP             .Read              ; And retry command
                SUB             [CS:.NumToRead],AL ; Read enough?
                JZ              .Done              ; Success!
; Note that the next calculation only works for a few number of sectors.
; If a larger number were required, the "Sectors per Track" would overflow,
; and we'd have to instead increase the Head count and zero the Sector count.
; If it overflowed again, we'd have to increase the Cylinder count and zero the
; Head and Sector counts. All too hard if we don't really need to do it,
; especially since determining what constitutes "overflow" is tricky!
; Finally, the "memory address to load to" calculation would overflow, limiting
; the final result to 64K maximum anyway
                ADD             CL,AL              ; Fewer sectors to read...
; Turn the Number Sectors Read value into a pointer offset:
                IMUL            AX,BIOS.Disk.Sector.Size
                ADD             BX,AX              ; Further in buffer...
                MOV             AH,BIOS.Disk.Fn.Reset
                INT             BIOS.Disk.Int      ; BIOS Reset
                DEC             SI                 ; One less try...
                JNS             .Attempt           ; (Leaves Z clear on last error!)
                JMP             $                  ; ***STOP!***
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