lDOS boot documentation

2020 by C. Masloch. Usage of the works is permitted provided that this instrument is retained with the works, so that any entity that uses the works is notified of this instrument. DISCLAIMER: THE WORKS ARE WITHOUT WARRANTY.

This document has been compiled on 2022-11-11.

1: lDOS boot protocols #

1.1 Sector to iniload protocol #

The iniload kernel is loaded to an arbitrary segment. The segment must be at least 60h. Common choices are 60h, 70h, and 200h. At least 1536 bytes of the file must be loaded. Current loaders will load at least 8192 bytes if the file is as large or larger than that. The entrypoint is found by applying no segment adjustment (0) and choosing the offset 400h (1024).

1.1.1 File properties #

The file must be at least 4096 bytes long. This is now required, beyond the former lower bound of 1536 bytes, to support an optimisation of the FAT12 and FAT16 boot sector loaders. The lDebug loader and the FAT32+FSIBOOT loader currently retain the 1536 bytes limit.

The file may allow multi-use as a flat .COM format executable, flat .SYS format device driver, or MZ .EXE format executable and/or device driver. It is also valid to append arbitrary sized data such as a .ZIP archive.

The file needs to be placed in the root directory for the boot sector loaders. The lDebug loader allows to load a file from any subdirectory and this is also allowed. The file may be fragmented in any part. The file data may be located anywhere in the file system. The supported cluster sizes should be between 32 Bytes and 2 MiB, inclusive. The sector size should be between 32 Bytes and 8 KiB, inclusive.

1.1.2 Signatures #

At offset 1020 (3FCh) there is the signature ‘lD’. Behind that there are two bytes with printable non-blank ASCII codepoints. Currently the following signatures are defined:

‘lDOS’
lDOS kernel (not yet in use)
‘lDRx’
RxDOS kernel
‘lDFD’
FreeDOS kernel wrapped in iniload (fdkernpl.asm)
‘lDeb’
lDebug
‘lDDb’
lDDebug (debuggable lDebug)
‘lDbC’
lCDebug (conditionally debuggable lDebug)
‘lDTP’
lDOS test payload kernel (testpl.asm)
‘lDTW’
lDOS test result writer kernel (testwrit.asm)

1.1.3 Load Stack Variables (LSV) #

Under this protocol, the pointer ‘ss:bp’ is passed. It points to a boot sector with (E)BPB. The stack pointer must be at most ‘bp - 10h’. Below the pointed to location there live the Load Stack Variables. These follow this structure: 

        struc LOADSTACKVARS, -10h
lsvFirstCluster:        resd 1
lsvFATSector:           resd 1
lsvFATSeg:              resw 1
lsvLoadSeg:             resw 1
lsvDataStart:           resd 1
        endstruc
lsvFirstCluster
(FAT12, FAT16) Low word gives starting cluster of file. High word uninitialised.
(FAT32) Dword gives starting cluster of file.
(else) Should be zero.
lsvFATSector
(FAT16) Low word gives loaded sector-in-FAT. -1 if none loaded yet. High word uninitialised.
(FAT32) Dword gives loaded sector-in-FAT. -1 if none loaded yet.
(FAT12, else) Unused.
lsvFATSeg
(FAT16, FAT32) Word gives segment of FAT buffer if word/dword [lsvFATSector] != -1.
(FAT12) Word gives segment of FAT buffer. Zero if none. Otherwise, buffer holds entire FAT data, up to 6 KiB.
lsvLoadSeg
Word points to segment beyond last loaded paragraph. Allows iniload to determine how much of it is already loaded.
lsvDataStart
Dword gives sector-in-partition of first cluster's data.

An LSV extension allows to pass a command line to the kernel. The stack pointer must be at most ‘bp - 114h’ then. This follows the structure like this: 

lsvclSignature          equ "CL"
lsvclBufferLength       equ 256

        struc LSVCMDLINE, LOADSTACKVARS - lsvclBufferLength - 4
lsvCommandLine:
.start:         resb lsvclBufferLength
.signature:     resw 1
lsvExtra:       resw 1
        endstruc
lsvCommandLine.start
Command line buffer. Contains zero-terminated command line string.
lsvCommandLine.signature
Contains the signature value ‘CL’ if command line is given.
lsvExtra
Used internally by iniload. Space for this must be reserved when passing a command line.

If no command line is passed then either the stack pointer must be ‘bp - 10h’, or ‘bp - 12h’, or the word in the lsvCommandLine.signature variable (word [ss:bp - 14h]) must not equal the string ‘CL’.

1.1.4 Memory map #

The initial loader part that is loaded must be loaded at above or equal to linear 00600h. The FAT buffer segment (if used) must also be stored at above or equal to linear 00600h. The stack (which should extend at least 512 bytes below ‘ss:bp’) and boot sector (pointed to by ‘ss:bp’, at least 512 bytes length) should also be stored at above or equal to linear 00600h.

There is an additional memory area, the Low Memory Area top reservation, which should be unused by the load protocol at handoff time but be at least 20 KiB in size. It is located below the usable Low Memory Area top. That is, directly below the EBDA, RPL-reserved memory, video memory, or otherwise UMA. This area is reserved in order to facilitate initial loader operation.

None of the memory areas may overlap. This does not include the FAT buffer in case it is uninitialised.

1.1.5 Load filename in the boot sector #

The boot sector may be expected to contain a valid 8.3 format (blank-padded FCB) filename in the area of the boot sector starting behind the (E)BPB, extending up to below the boot sector signature word with value AA55h (at offset 510 in the boot sector). This name should not contain blanks other than trailing in the file name portion or trailing in the file extension portion. It should consist of printable ASCII codepoints. That is, byte values between 20h and 7Eh inclusive. It should not consist of eleven times the same byte value. Additional FAT Short File Name restrictions may be assumed.

Although a loader should not depend on this for crucial operation, it may want to detect the kernel name it was presumably loaded from for informational or optional purposes. The canonical implementation of this is currently the function ‘findname’ in the testpl.asm test payload kernel. It is found within the ldosboot repo. This handling is based on the function of the same name in the instsect application.

1.2 Iniload to payload protocol #

The payload is loaded to an arbitrary segment. The segment must be at least 60h. The entire payload must be loaded. The size of the payload is determined at iniload build time. The entrypoint is found by applying a segment adjustment and choosing an offset. The segment adjustment is specified at iniload build time by the numeric define _EXEC_SEGMENT (default 0), and the offset by the define _EXEC_OFFSET (default 0).

1.2.1 Extended BIO Parameter Block (EBPB) #

Above the LSV, ss:bp points to an EBPB and surrrounding boot sector. Note that this is always a FAT32-style EBPB. If the filesystem that is loaded from is not FAT32, and is therefore FAT16 or FAT12, then the FAT16/FAT12 BPBN structure is moved up. It is placed where the FAT32 BPBN is usually expected. In this case, the entire boot sector contents behind the BPBN are also moved up by the size of the FAT32-specific fields. The FAT32-specific fields are filled with zeros, except for the FAT32 ‘sectors per FAT’ field. It is filled with the contents of the FAT16/FAT12 ‘sectors per FAT’ field.

1.2.2 Load Stack Variables (LSV) #

Refer to section 1.1.3.

1.2.3 Load Data 1 (LD) #

Below the LSV, iniload passes the LOADDATA (1) structure. 

        struc LOADDATA, LOADSTACKVARS - 10h
ldMemoryTop:    resw 1
ldLoadTop:      resw 1
ldSectorSeg:    resw 1
ldFATType:      resb 1
ldHasLBA:       resb 1
ldClusterSize:  resw 1
ldParaPerSector:resw 1
ldLoadingSeg:   resw 1
ldLoadUntilSeg: resw 1
        endstruc
ldMemoryTop
Word. Segment pointer to behind usable memory. Points at the first of the EBDA, RPL-reserved memory, or video memory or otherwise UMA. Indicates how much memory may be used by a typical kernel. (lDebug detects the EBDA to move that below where it installs.)
ldLoadTop
Word. Segment pointer to lowest lDOS boot memory in use. All memory between linear 600h and the segment indicated here is usable by the payload. Only the payload itself is stored in this area. The other buffers, stack, and structures passed by iniload must live above this segment.
ldSectorSeg
Word. Segment pointer to an 8 KiB transfer buffer. It is insured that this buffer does not cross a 64 KiB boundary. This may be needed by some disk units. The buffer is not initialised to anything generally.
ldFATType
Byte. Indicates length of FAT entry in bits. 12 indicates FAT12, 16 FAT16, 32 FAT32. It is planned to allow zero for non-FAT filesystems.
ldHasLBA
Byte. Only least significant bit used. Bit on indicates LBA extensions available for the load disk unit. Bit off indicates LBA extensions not available.
ldClusterSize
Word. Contains amount of sectors per cluster. Unlike the byte field for the same purpose in the BPB, this field can encode 256 (EDR-DOS compatible) without any masking. May be given as zero for non-FAT filesystems.
ldParaPerSector
Word. Contains amount of paragraphs per sector. Must be a power of two between 2 (32 B/s) and 200h (8192 B/s). May be given as zero for non-FAT filesystems.
ldLoadingSeg
Word. Internally used by iniload. Available for re-use by payload.
ldLoadUntilSeg
Word. Internally used by iniload. Available for re-use by payload.

1.2.4 Load Command Line (LCL) #

Below the LOADDATA structure, iniload passes the LOADCMDLINE structure. 

lsvclBufferLength       equ 256

        struc LOADCMDLINE, LOADDATA - lsvclBufferLength
ldCommandLine:
.start:         resb lsvclBufferLength
        endstruc

This buffer is always initialised to an ASCIZ string. At most 255 bytes may be initialised to string data. At most the 256th byte is a zero.

If the first word of the buffer is equal to 0FF00h, that is there is an empty command line the terminator of which is followed by a byte with the value 0FFh, then no command line was passed to iniload. Currently lDebug can pass a command line to iniload when loading with its lDOS, RxDOS.2, RxDOS.3, or FreeDOS protocols. When iniload is loaded as a Multiboot1 or Multiboot2 specification kernel, it is also assumed that a command line can be passed.

Source Control Revision ID #

hg 7bcae97a16aa, from commit on at 2022-11-11 17:00:10 +0100

If this is in ecm's repository, you can find it at https://hg.pushbx.org/ecm/ldosboot/rev/7bcae97a16aa