00DB Critical Error Handler

This is the Turbo Pascal's critical error handler that replaces DOS's.

These registers are set upon entry to this handler:

The stack on entry

Index	Contents
SP	DOS IP Register
SP+02	DOS CS Register
SP+04	DOS Flags Register
SP+06	Original Caller's AX register
SP+08	Original Caller's BX register
SP+0A	Original Caller's CX register
SP+0C	Original Caller's DX register
SP+0E	Original Caller's SI register
SP+10	Original Caller's DI register
SP+12	Original Caller's BP register
SP+14	Original Caller's DS register
SP+16	Original Caller's ES register
SP+18	Original Caller's IP register
SP+1A	Original Caller's CS register
SP+1C	Original Caller's Flags register

SYS00DB: STI

Enable interrupts.

SYS00DC: ADD SP,+06
SYS00DF: POP AX

Adjust stack and get original caller's AX register.

SYS00E0: AND DI,+1F
SYS00E3: ADD DI,0096

Adjust error code (clear upper bits 5-15) and add 150 (96h) ???.

SYS00E7: CMP AH,39
SYS00EA: JNB 00EF

Determine which DOS service was called when critical error occured. Check if any action can be made to address this.

SYS00EC: MOV DI,FFFF

Set DI = -1 (FFFFh), i.e. error in DOS service cannot be handled (??).

SYS00EF: PUSH DI

Set original caller's AX by pushing DI to the stack.

SYS00F0: MOV AH,54
SYS00F2: INT 21

Get read-after-write verify flag DOS INT 21h AH = 54h.

On return:

It seems that the result is unused.

SYS00F4: MOV BP,SP

Use BP to address the stack.

SYS00F6: OR BYTE PTR [BP+16],01

Set original caller's CF flag to 1.

SYS00FA: POP AX
SYS00FB: POP BX
SYS00FC: POP CX
SYS00FD: POP DX
SYS00FE: POP SI
SYS00FF: POP DI
SYS0100: POP BP
SYS0101: POP DS
SYS0102: POP ES

Restore all registers to original caller's.

SYS0103: IRET

Return to original caller.