starflight reverse
1.0.0
Back in the 80ths, an unknown company called Binary Systems published the game Starflight. The game puts the player in the role of a starship captain sent to explore the galaxy. There is no set path, allowing players to switch freely between mining, ship-to-ship combat, and alien diplomacy. The broader plot of the game emerges slowly, as the player discovers that an ancient race of beings is causing stars to flare and destroy all living creatures. The game has been widely praised by both contemporary and modern critics, and is one of the earliest instances of a sandbox game. The game influenced the design of numerous other games for decades after its release.
To find out more about the game check the following links:
You can buy the game at GoG
The first time I heard about the game I wanted to play it. However, I was too young and could not speak English. 20 later I tried again and it was a very pleasant experience. The exploration is fun, the storyline is epic and ends with a surprise, that is one of the best I have experienced. Sure, the game hasn't aged well, but you can feel the devotion of the developers to the game. There’s an art aspect to this game as well as a craftman’s attention to detail.
As much as playing this truly amazing game is fun, so is reverse engineering this game. You follow in the footsteps of the developers and experience their thought processes as if it were the year 1985 again. For this game expect the unexpected. Normally when you reverse engineer such an old game you have to receive ten thousands of lines of pure assembler code, which you can analyze with the usual tools such as IDA Pro. But not this time. Actually for this game you can throw the usual tools away. They are useless. You are on your own. The reason is that Starflight was written in Forth, a language I barely knew about.
Forth is the language with the ultimate minimalism regarding syntax. There is no more syntax than the space between "words". You can write a Forth reader and interpreter basically in a few lines of code.
In a modern language you write something like
print(2+3)to print the result of 2+3. In Forth however it looks like this.
2 3 + .Forth is a stack machine, with a reverse Polish notation. The interpretation is as follows
The syntax is simple and the interpreter is simple. "2", "3", "+" and "." are just called "words". There is no syntactic distinction between data and code. Certainly a language that lived up to the limitations of the early home computers.
When you dissect the executable STARFLT.COM it reveals some fantastic internals
As explained above Forth is a stack machine. As coding mechanic it uses indirect threading, a very space efficient method to store your compiled code. Threaded code has a form that essentially consists entirely of calls to subroutines. Indirect threading uses pointers to locations that in turn point to machine code.
Let's say your instruction pointer points to the address 0x1000 and contains the 16-Bit value Read16(0x1000)=0x0f72.
0x1000: dw 0x0f72The value 0x0f72 is the coded equivalent of the Forth word '+'. Remember the description above. The word '+' pops the last two stack entries, adds them together and pushes the result back on top of the stack. According to indirect threading this 16-Bit value 0x0f72 is a pointer to a location that in turn points to machine code. When you read the memory content Read16(0x0f72) you get the pointer to 0x0f74. And indeed, when you look at this memory location and disassemble it, you receive the following
0x0f72: dw 0x0f74
0x0f74: pop ax
0x0f75: pop bx
0x0f76: add ax, bx
0x0f78: push ax
0x0f79: lodsw
0x0f7a: mov bx, ax
0x0f7c: jmp word ptr [bx]The first four instructions perform exactly the operations that the word "+" should perform. The last three assembler instructions starting from the "lodsw" increase the instruction pointer and jump to the next code.
Let us go on. Now the instruction pointer points to 0x1002
0x1002: dw 0x53a3Reading the address 0x53a3 reveals
0x53a3: dw 0x1d29
0x53a5: dw 0x0001and the corresponding code
0x1d29: inc bx
0x1d2a: inc bx
0x1d2b: push bx
0x1d2c: lodsw
0x1d2d: mov bx,ax
0x1d2f: jmp word ptr [bx]At this time the register bx contains the word address 0x53a3. So this code just pushes the address 0x53a5 on top of the stack. What we have done is to provide the program a pointer to a variable. The variable has the content 0x0001. The Forth word '@' would pop the address from the stack, reads its content and pushes it back on the stack.
So far I could identify 6256 words that contain either code or data.
And that's actually all you need to know about the code structure. As you can see this can be a space efficient encoding, but speedwise it is a catastrophe. Every few machine code instructions you have to jump to a different code block.
The equivalent of indirect threading in C would look like this.
uint16_t instruction_pointer = start_of_program_pointer;
void Call(uint16_t word_adress)
{
// the first two byte of the word's address contain
// the address of the corresponding code, which must be executed for this word
uint16_t code_address = Read16(word_address);
switch(code_address)
{
.
.
.
case 0x0f74: // word '+'
Push16(Pop16() + Pop16());
break;
.
.
.
}
}
void Run()
{
while(1)
{
uint16_t word_address = Read16(instruction_pointer);
instruction_pointer += 2;
Call(word_address);
}
}The code executed for a specific word has access to 5 major variables (16-Bit)
The disassember transpiles the FORTH code into C-style code.. Most of the transpiled code compiles. To understand what the program does take a look at the following table. It takes the "bytecode" (which are mainly 16-Bit pointers) as input and transforms it into C.
Forth code:
: .C ( -- )
Display context stack contents.
CR CDEPTH IF CXSP @ 3 + END-CX
DO I 1.5@ .DRJ -3 +LOOP
ELSE ." MT STK"
THEN CR ;
EXITTransformation:
| 16-Bit Pointers | FORTH | C |
|---|---|---|
| : .C ( -- ) | void DrawC() { |
|
unsigned short int i, imax; |
||
| 0x0642 | CR | Exec("CR"); |
| 0x75d5 | CDEPTH | CDEPTH(); |
| 0x15fa 0x0020 | IF | if (Pop() != 0) { |
| 0x54ae | CXSP | Push(Read16(pp_CXSP) + 3); |
| 0xbae | @ | |
| 0x3b73 | 3 | |
| 0x0f72 | + | |
| 0x4ffd | END-CX | Push(Read16(cc_END_dash_CX)); |
| 0x15b8 | DO | i = Pop(); |
imax = Pop(); |
||
do { |
||
| 0x50e0 | I | Push(i); |
| 0x4995 | 1.5@ | _1_dot_5_at_(); |
| 0x81d5 | .DRJ | DrawDRJ(); |
| 0x175d 0xfffd | -3 | Push(-3); |
| 0x155c 0xffff | +LOOP | int step = Pop(); |
i += step; |
||
if (((step>=0) && (i>=imax)) || ((step<0) && (i<=imax))) break; |
||
} while(1); |
||
| 0x1660 0x000b | ELSE | } else { |
| 0x1bdc | " MT STK" | PRINT("MT STK", 6); |
| 0x06 | ||
| 0x4d | 'M' | |
| 0x54 | 'T' | |
| 0x20 | ' ' | |
| 0x53 | 'S' | |
| 0x54 | 'T' | |
| 0x4b | 'K' | |
| THEN | } |
|
| 0x0642 | CR | Exec("CR"); |
| 0x1690 | EXIT | } |
The game comes in 3 Files
Content of STARA.com
| entry | size | description |
|---|---|---|
| DIRECTORY | 4096 | contains directory of STARA and STARB |
| ELO-CPIC | 4816 | |
| GAZ-CPIC | 3120 | |
| MEC-CPIC | 2848 | |
| MYS-CPIC | 6064 | |
| NOM-CPIC | 1136 | |
| SPE-CPIC | 1888 | |
| THR-CPIC | 2480 | |
| VEL-CPIC | 4672 | |
| VPR-CPIC | 1248 | |
| MIN-CPIC | 2096 | |
| SPLASH | 16384 | Picture |
| MED-PIC | 2048 | Picture |
| PHAZES | 6144 | |
| HUM-PIC | 480 | Picture |
| VEL-PIC | 432 | Picture |
| THR-PIC | 272 | Picture |
| ELO-PIC | 608 | Picture |
| AND-PIC | 640 | Picture |
| SAVE | 124000 | |
| MUSIC | 4960 | Code Overlay |
| EARTH | 1152 | Map of the planet earth |
| GALAXY | 6304 | |
| CREDITS | 16384 | picture |
| COP-CPIC | 2928 | |
| FONTS | 768 | |
| CGA | 3600 | Machine Code routines for the CGA graphics card |
| EGA | 3600 | Machine Code routines for the EGA graphics card |
Content of STARB.COM
| entry | size | description |
|---|---|---|
| DIRECTORY | 4096 | contains directory of STARA and STARB |
| INSTANCE | 150528 | Tree structure with most of the content of the game |
| BOX | 1024 | Table |
| BANK-TRANS | 144 | Table |
| CREWMEMBER | 128 | Table |
| VESSEL | 1936 | Table |
| ELEMENT | 544 | Table |
| ARTIFACT | 1584 | Table |
| PLANET | 1360 | Table |
| SPECIMEN | 448 | Table |
| BIO-DATA | 448 | Table |
| TPORT-PIC | 2416 | Picture |
| BPORT-PIC | 3984 | Picture |
| ANALYZE-TEXT | 3200 | Table |
| BUTTONS | 944 | Table |
| ICON1:1 | 912 | |
| ICON1:2 | 912 | |
| ICON1:4 | 912 | |
| ICON-NAME | 736 | |
| DPART-OV | 1552 | Code Overlay |
| REGIONS | 176 | Table |
| CREATURE | 17024 | Table |
| CHKFLIGHT-OV | 960 | Code Overlay |
| FRACT-OV | 4640 | Code Overlay |
| ICONP-OV | 832 | Code Overlay |
| SITE-OV | 1888 | Code Overlay |
| HYPERMSG-OV | 4112 | Code Overlay |
| GPOLY | 368 | |
| FACET | 288 | |
| VERTEX | 416 | |
| BLT-OV | 864 | Code Overlay |
| MISC-OV | 1440 | Code Overlay |
| BANK-OV | 1520 | Code Overlay |
| ASSCREW-OV | 2800 | Code Overlay |
| PERSONNEL-OV | 4192 | Code Overlay |
| SHIPGRPH-OV | 2112 | Code Overlay |
| CONFIG-OV | 3072 | Code Overlay |
| TDEPOT-OV | 4800 | Code Overlay |
| PORTMENU-OV | 3120 | Code Overlay |
| VITA-OV | 3552 | Code Overlay |
| HP-OV | 4832 | Code Overlay |
| LP-OV | 5280 | Code Overlay |
| SENT-OV | 4784 | Code Overlay |
| TV-OV | 3472 | Code Overlay |
| COMM-OV | 7232 | Code Overlay |
| COMMSPEC-OV | 2864 | Code Overlay |
| SEED-OV | 2400 | Code Overlay |
| LISTICONS | 720 | Code Overlay |
| MOVE-OV | 3808 | Code Overlay |
| ENGINEER | 2320 | Code Overlay |
| DOCTOR | 1280 | Code Overlay |
| ORBIT-OV | 6640 | Code Overlay |
| CAPTAIN | 5952 | Code Overlay |
| SCIENCE | 3952 | Code Overlay |
| NAVIGATR | 880 | Code Overlay |
| SHIPBUTTONS | 1984 | |
| MAP-OV | 4160 | Code Overlay |
| HYPER-OV | 7168 | Code Overlay |
| ANALYZE-OV | 2560 | Code Overlay |
| LAUNCH-OV | 1360 | Code Overlay |
| FLUX-EFFECT | 464 | |
| OP-OV | 4400 | Code Overlay |
| ITEMS-OV | 6016 | Code Overlay |
| LSYSICON | 752 | |
| MSYSICON | 448 | |
| SSYSICON | 176 | |
| BEHAV-OV | 5360 | |
| CMAP | 1008 | |
| INSTALL | 800 | |
| HEAL-OV | 1232 | Code Overlay |
| REPAIR-OV | 1696 | Code Overlay |
| GAME-OV | 5920 | Code Overlay |
| PLSET-OV | 2400 | Code Overlay |
| MAPS-OV | 2240 | Code Overlay |
| VES-BLT | 4528 | |
| STORM-OV | 1232 | Code Overlay |
| COMPOUNDS | 176 | Table |
| IT-OV | 1936 | Code Overlay |
| COMBAT-OV | 6192 | Code Overlay |
| DAMAGE-OV | 2752 | Code Overlay |
| LAND-OV | 1088 | Code Overlay |
| PSTATS | 64 | Table |
| STP-OV | 1440 | Code Overlay |
Put the files of the original Starflight game into the folders starflt1-in and starflt2-in and run make. You should get two executables (disasOV1 and disasOV2), which produces the content in the folders starflt1-out and starflt2-out. The generated output is part of this repository.
