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====== DOSKRNL ======

DOSKRNL is a specialized FreeDOS kernel changed to support DOS in MVM. Mostly all FAT code removed as well as CONFIG.SYS parser. So it smaller in compare with original FreeDOS kernel. Also image format changed because different load logic.

===== Initialization =====

DOSKRNL loaded at address differ from standard DOS 0060:0000 address. osFree DOSKRNL expect unknown load address, but in low address of conventional memory.

On DOSKRNL startup registers are following:

  * CS:IP - starting point
  * SS:BP - pointer to the DOSKRNL init structure
  * SS:SP - stack pointer (size around 800 bytes)

DOSKRNL init structure:

^ Offset ^ Size ^ Description ^
| 0      | 2    | First free segment after DOSKRNL |
| 2      | 2    | Size of memory - first free segment (paragraphs) |
| 4      | 2    | Size of init area (paragraphs)|
| 6      | 2    | Value of BREAK setting | 
| 8      | 2    | Value of DOS setting | 
| 10     | 4    | Far pointer to list of DOS DEVICE setting |
| 14     | 4    | Far pointer to SHELL (filepath only) |
| 18     | 4    | Far pointer to SHELL (arguments) |
| 22     | 4    | Far pointer to linked list of VDD |
| 26     | 1    | Current drive (1-A, 2-B, 3-C, ...)|
| 27     | 1    | Boot drive (1-A, 2-B, 3-C, ...) |
| ????   | ???  | ???? |

DEVICES is ASCIIZ string with list of DOS devices to be loaded divided by 0AH

SHELL is ASCIIZ string.

SHELL arguments is ASCIIZ string (first byte is as string length, string ends with 0dh)

VDD is linked list of structures:

VDDs linked list in standard DOS Device drivers format. Entry can be placed in HMA, so need to be sure A20 enabled for such entries.

^ offset ^ Size ^ Description ^
|  0     |  4  | Pointer to next device (Must be set to -1 for last device)  |
|  4     |  2  | Attributes                      |
| :::    | ::: | Bit 15 = 1 if char device 0 if blk     |
| :::    | ::: |  if bit 15 is 1                        |
| :::    | ::: |    Bit 0 = 1 if Current STDIN device     |
| :::    | ::: |    Bit 1 = 1 if Current STDOUT output     |
| :::    | ::: |    Bit 2 = 1 if Current NULL device     |
| :::    | ::: |    Bit 3 = 1 if Current CLOCK device      |
| :::    | ::: |    Bit 4 = 1 if SPECIAL                |
| :::    | ::: |  Bit 14 is the IOCTL bit               |
| :::    | ::: |  Bit 13 is the NON IBM FORMAT bit      |
|  6     |  2  | Pointer to Device strategy entry point |
|  8     |  2  | Pointer to Device interrupt entry point |
|  10    |  8  | character device name field. Character devices set a device name. For block devices the first byte is the number of units |

Memory map on DOSKRNL start:

| Init area         | [BP+0+BP+2-BP+4]:0 |
| Free              | [BP+0]:0 |
| DOSKRNL           | ???:0 |
| Some data (VDD entries was found here) | ??? |
| CMOS Data         | 40:0 |
| Interrupt vectors | 0:0  |

As in most DOS kernels, DOSKRNL moves initialization code to higher conventional memory. But uses init structure, not BIOS INT 12h, for memory information.

DOSKRNL search and initialize XMS driver and moves some parts to HMA. Most other initialization things is same. DOSKRNL initializes standard device drivers and adds and initializes VDDs, passed via init structure. Some things (at least COMMAND.COM, device drivers) also passed via init structure. not CONFIG.SYS. At the present time interface not investigated well.

===== CONFIG.SYS =====

CONFIG.SYS device information and some other settings not parsed by DOSKRNL, but passed via init structure. Here list of DOSKRNL options:

  * RMSIZE
  * DEVICE
  * SHELL
  * DOS
  * BREAK

todo add more info here

Also most of these setting can be adjusted via DOS Properties (aka DOS Settings). Refer [[https://archive.org/download/gg243731/gg243731_OS2_V2_0_Vol_2_DOS_and_Windows_Environment.pdf|OS/2 Version 2.0 Volume 2: DOS and Windows Environment]] for more information.

===== Properties =====

  * COM_DIRECT_ACCESS
  * COM_HOLD
  * COM_RECEIVE_BUFFER_FLUSH
  * COM_SELECT
  * DOS_AUTOEXEC
  * DOS_BACKGROUND_EXECUTION
  * DOS_BREAK
  * DOS_DEVICE
  * DOS_FCBS
  * DOS_FCBS_KEEP
  * DOS_FILES
  * DOS_HIGH
  * DOS_LASTDRIVE
  * DOS_RMSIZE
  * DOS_SHELL
  * DOS_STARTUP_DRIVE
  * DOS_UMB
  * DOS_VERSION
  * DPMI_DOS_API
  * DPMI_MEMORY_LIMIT
  * DPMI_NETWORK_BUFF_SIZE
  * EMS_FRAME_LOCATION
  * EMS_HIGH_OS_MAP_REGION
  * EMS_LOW_OS_MAP_REGION
  * EMS_MEMORY_LIMIT
  * HW_NOSOUND
  * HW_ROM_TO_RAM
  * HW_TIMER
  * IDLE_SECONDS
  * IDLE_SENSITIVITY
  * INT_DURING_IO
  * KBD_ALTHOME_BYPASS
  * KBD_BUFFER_EXTEND
  * KBD_CTRL_BYPASS
  * KBD_RATE_LOCK
  * MEM_EXCLUDE_REGION
  * MEM_INCLUDE_REGION
  * MOUSE_EXCLUSIVE_ACCESS
  * PRINT_SEPARATE_OUTPUT
  * PRINT_TIMEOUT
  * SESSION_PRIORITY
  * VIDEO_8514A_XGA_IOTRAP
  * VIDEO_FASTPASTE
  * VIDEO_MODE_RESTRICTIONS
  * VIDEO_ONDEMAND_MEMORY
  * VIDEO_RETRACE_EMULATION
  * VIDEO_ROM_EMULATION
  * VIDEO_SWITCH_NOTIFICATION
  * VIDEO_WINDOW_REFRESH
  * XMS_HANDLES
  * XMS_MEMORY_LIMIT
  * XMS_MINIMUM_HMA 

===== API =====

According different sources IBM DOSKRNL implements API on level of MS-DOS 5.x. osFree DOSKRNL supports API as in FreeDOS kernel. More information about implemented functions available [[en:docs:dos:api|here]]. DOSKRNL also implements some extensions to API mostly aimed to communicate with host system.

===== Supervisor calls =====

DOSKRNL uses [[en:docs:mvm:api|SVC]] interface to call host functions.