going to moscow loads the same byte. strange.
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they seek to position X
read 1 byte
read 99 more bytes
then seek to position X+100
now if you know how both math and random access files work, you'll realize something the programmers of Where in the World is Carmen Sandiego? Enhanced (1990, DOS) did not:
THEY'RE SEEKING TO THE POSITION THEY'RE ALREADY AT
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I tried to corrupt the image to see if that'd tell me anything about how it was encoded, and it told me to put my hard drive back in.
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the way this game does the investigations is interesting.
so the basic gameplay is that you're in location X, you get 3 hints, which lead you to location Y, where the whole process repeats.
But if you savescum to experience the same pursuit again, they'll always go through the same places... but if you don't get the hints, they won't be there.
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like the hints will always tell you to go to sri lanka, but if you go there without first having heard those hints, then he won't be in sri lanka
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Hah! the game apparently calculates some info ahead of time, but only a few steps. I changed who the suspect was by memory editing, and it didn't take effect... until I got to the third location.
Since I went from a robbery by Fast Eddie B to one by Merey LaRoc, it means the pronouns changed when I got to London.
Congrats on coming out as a trans woman, Merey.
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ok I ran my dosspin tool to gibberish every byte of the save game file (it's only 102 bytes, so this is easy!) and none of them change where you start. very interesting... I'm guessing either the values are spread out too much for my gibberishing to reach, or you need to modify multiple bytes at once
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huh, I found a hidden(?) key: if you hold down either shift, it skips all the pauses in the printing. so it goes at MAX CPU SPEED
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ahh good. it's always fun to find code that looks like:
do{
while(variable!=0);
some one has a custom tick handler that's permutating a global!
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can't be threading, this is 16-bit DOS. There is no threading.
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looking at interrupts, and I think I found a bug.
they set handlers for various CPU errors, but they accidentally set 10 (COPROCESSOR ERROR) twice, instead of the 05 (BOUND check)/10 (COPRPOCESSOR) interrupts they save
someone copy-pasted and missed a bit
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I finally found the two helper functions they use to get and set vectors!
all the 30 other places I've seen them set/get vectors, they do it manually, but hey, maybe they use the helpers too
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could also be that this is a compiler-provided bit of code, which is left in because the runtime needs it, or they just didn't eliminate dead code
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okay I've figured out there's a shared format they're using here. it chunks the file into chunks, which have a 16-bit ID (unique per file, but not globally), an offset, and 16-bit length
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so like, midisnd.dat will have 12 entries, and the first 11 are 200-500 bytes each, and then the last is 3k.
presumably it's each song and then some config info?
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cities.dat is very interesting. There's 30 cities in total, but 491 entries in it!
So they must be doing something odd there, that doesn't divide equally. Maybe one city-chunk gives IDs of the others?
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idea for a test: it's easy to spot which chunk in a city is the image, because it's the biggest. Here's a way to determine if it's looking up by IDs or offsets/indices: swap the IDs of two images
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darn. turns out you can't just renumber the chunks, because they have to be in increasing order.
so maybe I just need to leave the chunk indexes as is, and instead of moving the entries around, I move where they're pointing?
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Bingo! I'm in Athens, but I'm seeing the image for Baghdad, and apparently with the Baghdad palette?
So one of these other chunks must be the palette for a city. Or it selects from a selection of palettes? Maybe they've just got a couple defined.
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okay I figured out the cities.dat IDs:
They're all 1XXYY (in decimal):
XX is the city number (0-29), YY is the sub-chunk-id.
So like:
YY=0: City name
YY=2: City image.
They go between 00 and 22, and not all numbers need to be present.
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hmm, reading a buffer and then summing all the values of the bytes in it.
suspicious behavior.
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okay I think it has a very simple 1-byte CRC check on the chunks, which are optionally not run.
I can't make the math work but I'm reasonably sure that's what it is
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okay they're using a blit that's UI-aware, so it starts the coordinate system at (1,13). Fun!
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looking into the blitting code I managed to steal the world map out of RAM
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ugh. TODO for my eventual Good DOS Debugger:
Instant Video display.
I don't know exactly how DOSBox-X is doing it, but while single-stepping the debugger, the display never updates. I can dump the ram at A000:0000 and see what updated, but not on the screen in DOSBox
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found a suspicious array, which goes:
[
(-1,0),
(-1,1),
(0,1),
(1,1),
(1,0),
(1,-1),
(0, -1),
(-1,-1),
(0,0)
]
POP QUIZ: why does the font renderer need this array? how are they being "lazy" with this array?
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there's also this code in the for-loop that steps through this array:
if index==8:
color=white
else:
color=black
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@dividuum got it:
they're drawing the font 9 times, offset in each of the 8 directions, and in black. then they draw it in white, with no offset.
It's a pixel-outliner! By drawing their pixel font offset in each direction, they get a black outline on their font.
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The Answer to the DRM questions for Where in the world is Carmen Sandiego? Enhanced (DOS, 1990) are, in no particular order:
23
Kent
dragon
calcium
1796
Warren
revenue
1792
Willard
1937
Crater
Tanzania
Hartford
Duluth
London
Gem
Silent
squeaker
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if ((0x80 >> ((byte)local_4 & 7) &
(int)(char)*(byte *)((int)((int *)param_1 + 1) + (local_4 >> 3))) != 0) {
COULD YOU USE SOME MORE CASTS MAYBE?
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oh it's because ghidra's near/far pointer support is shit.
I had param2 defined as a byte32 and it was casting it to a byte before using it
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if I define it as byte* and let the calling convention implicitly define it as 32bit, it doesn't do the cast
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well I found the decompression method.
as always, I hate it. decompression routines are probably my least favorite thing to reverse engineer
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I think this compression is specifically designed for ASCII text, which is annoying because they've also got compressed images... which probably use a DIFFERENT COMPRESSION!
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it looks like this chunk has length 256, which means 253 usable bytes, and it expands to 374 bytes.
Not the greatest compression. a little better than just doing 6-bit ASCII.
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it's some kind of shifting bit mask but it starts at encoding values in 4 bits, then it can increase (or decrease, I guess) based on the input stream.
then it has an output filter, where if the number specified wasn't 8 bits, it's actually an index into a predefined text table
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the predefined table starts with NUL, space, then:
aetonisrdlhugfcwypbmk,vSA.T'PMxBCIRGDWHqE-zNFKL0j:51YJ8\U?73Q;2!469
\r\nOVXZ()*+"#$%&<=>/@[]^_`
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given that the most comment symbols are near the beginning, this is presumably a sort of lazy huffman coding
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but I've got the predefined table, an input file, an output file, and now I need to write some python code to replicate this, hopefully without crying
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"vs ses oa is isgit's tc eital and largest t u anhtA ttggh os nnotosnhrdsmarosogdn ss drte tishoth's isdhsceohtsnthminder of isgit's t nuorhdhtpast\x00 geru is slightltsn oaller than ndhd na and is o nnsgtgstbtst oa dotlalssaaolootbiaoht Sal gh, sonuhvia and sl ghh\x00isgit, ontvdn ss nhsiaalgarsnadlfnaatawlarst oadrlhrs i is a rugged land dooousr'casrbhe nrdsgs fountainsnht iah"
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@foone so true
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