This is the Turbo Pascal’s critical error handler that replaces DOS’s.
These registers are set upon entry to this handler:
| 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.