When the executable MAIN.EXE is loaded in memory via DEBUG.EXE command, the following registers were assigned these values:
AX=FFFF BX=0000 CX=0B64 DX=0000 SP=4000 BP=0000 SI=0000 DI=0000
DS=075E ES=075E SS=07F4 CS=076E IP=0000 NV UP EI PL NZ NA PO NC
The absolute values for DS, ES, SS, and CS itself are not important. What is relevant are the distances from one another (see below).
CX is initialized to the file size minus the header (512 bytes):
CX = 3428 - 512 = 2916 (in Decimal)
CX = 0D64 - 200 = 0B64 (in Hexadecimal)
Stack size is around 16384 kilobytes (SP = 4000h).
As to why AX = FFFFh, it is still a mystery to me as of now and will be explored later.
When the program is loaded in memory, ES AND DS points to the Program Segment Prefix (PSP). The system library follows CODE segment immediately but is still aligned on paragraph (16 bytes) boundaries.
| Segment | Value | Distance from CODE segment | Distance calculation | size / hex / align / paragraphs + start = next |
|---|---|---|---|---|
| PSP | 075E | 016 paragraphs above CODE | 076E - 0010 = 075E | 00256 => 0100 => 0100 => 010 + 75E = 76E |
| CODE | 076E | 00023 => 0017 => 0020 => 002 + 76E = 770 | ||
| SYSTEM | 0770 | 002 paragraphs below CODE | 076E + 0002 = 0770 | 01426 => 0592 => 05A0 => 05A + 770 = 7CA |
| DATA | 07CA | 092 paragraphs below CODE | 076E + 005C = 07CA | 00668 => 029C => 02A0 => 02A + 7CA = 7F4 |
| STACK | 07F4 | 134 paragraphs below CODE | 076E + 0086 = 07F4 | 16384 => 4000 => 4000 => 400 + 7F4 = ??? |
Where Distance from CODE segment is calculated using:
(segment) - (code segment) = distance in paragraphs (1 paragraph = 16 bytes = 10h in hex)
Other calculations based on actual sizes of these segments as well as the rounded-off values are shown in the the last column of the table above. The rounded values are due to the assembler putting these segments in paragraph alignments. The location of the HEAP is calculated during the system library initialization.
Refer to MAIN.MAP to see where these values are obtained.
CODE0000: CALL SYS:0000
Initialize System Library: SYS:0000 System Library Initialization
CODE0005: PUSH BP
CODE0006: MOV BP,SP
Probably like other compiled high-level languages, Turbo Pascal has a lot of stack manipulation. This sequence of statements PUSH BP/MOV BP, SP is like a defining signature of compiled code. It’s likely that the main program is a callable function and receives parameters (e.g. argc, argv) on the stack right next to the return code. It is also possible that the loader or the parent program did a FAR JMP to CODE:0000.
BP is used often to address these parameters on the stack. In most cases Turbo Pascal compiler takes special care of BP and SP, while playing fast and loose with the other registers.
CODE0008: XOR AX,AX
CODE000A: CALL SYS:02CD
Checks the size of the stack with a call to SYS:02CD Check Stack. Exits with an error message if stack size is not sufficient or it is below the minimum.
CODE000F: POP BP
This balances the stack from PUSH BP/MOV BP, SP above. Again, this seems to be a defining feature of compiled code.
CODE0010: XOR AX,AX
CODE0012: CALL SYS:0116
Call SYS:0116 Ctrl-C / Int 23h Handler / Program Exit. AX contains the ExitCode. AX = 0 means there were no errors encountered.