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Booting from MS-DOS

Program Rttboot allows booting an RTTarget-32 program from MS-DOS. Rttboot.com is a small MS-DOS loader program which loads an RTB file given on its command line and transfers control to it. The file is read via DOS and all program entities are copied to their target addresses. Subsequently, a far jump to the application's boot code or boot vector is performed.

Rttboot cannot load and boot every RTTarget-32 program. In particular, the following restrictions apply:

• The computer must be running in real mode (i.e., not in virtual 8086 mode).
• No program entity with an associated image can overlap any address range currently occupied by DOS or Rttboot.com itself.

To satisfy the first requirement, no memory manager such as EMM386, QEMM, etc. may be loaded (EMM memory managers run the computer in virtual 8086 mode).

The second requirement will usually also not allow using an XMS memory manager such as HIMEM.SYS, since HIMEM will allocate all available extended memory. The only exception are RTTarget-32 programs which do not load any images into extended memory.

Apart from a suitably configured DOS system, an application to be loaded via Rttboot must also be configured correctly:

• Biosboot.exe must be used as boot code.
• Biosboot.exe and the boot data must be allocated within the first MB of address space.
• Memory allocated by DOS and Rttboot must not be used for program images.

Since Rttboot transfers control to the application, a boot vector is not required. If one is used, RTLoc will issue a warning that the vector is not located at a suitable address. If no boot vector is used, RTLoc will warn about a missing boot vector. Both warnings can safely be ignored. Locating a DiskBuffer can surpress the warning (and keep the program compatible with booting from disk).

DOS will typically occupy the first 64k of real-mode address space. Rttboot is a COM file and thus will occupy exactly another 64k. Therefore, on a minimal DOS system, it is sufficient to reserve the first 128k for DOS (note: these 128k can still be used for RAM remapping, or for entities not requiring an image). However, to be on the safe side, it is recommended to reserve at least 256k for DOS.

Example:

Region  NullPage         0    4k  RAM 
Region  DOSMem          4k  252k  RAM 
Region  LowMem        256k  256k  RAM 
Region  HighMem         1M    1M  RAM 

Region  MonoText    B0000h    4k  Device  ReadWrite 
Region  ColorText   B8000h    4k  Device  ReadWrite 
VideoRAM ColorText 

FillRAM HighMem 

Locate  BootCode    Biosboot.exe  LowMem 
Locate  BootData    BootData      LowMem  0 16 
Locate  DiskBuffer  DiskBuffer    LowMem 

Locate  PageTable   PageTable     HighMem 
Locate  Header      Header        HighMem 
Locate  NTSection   CODE          HighMem 
Locate  NTSection   DATA          HighMem 
Locate  Stack       Stack         HighMem 16k 
Locate  Heap        Heap          HighMem

This example uses the scarce conventional memory only for entities which must reside in real mode address space: boot code, boot data, and the (for DOS booting optional) disk buffer.

RTTarget-32 takes complete control of the hardware and thus cannot return to DOS. When the RTTarget-32 application terminates, the PC is rebooted.

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Program Rttboot

UEFI Booting