… atau dengan kata lain, cara paling bodoh untuk masuk ke dalam sistem terbenam.
Tonton beraksi di sini!
Matlamatnya mudah. Tulis beberapa kod dalam C atau C++, dan boleh laksanakan dalam Scratch. Secara jujur, saya baru saja mendapati idea itu agak lucu: salah satu bahasa pengaturcaraan terpantas dalam salah satu bahasa yang paling perlahan. Saya mempunyai perasaan ia mungkin, tetapi saya tidak pasti bagaimana. Dalam proses itu, saya belajar lebih banyak tentang bahasa pemasangan, memori proses dan fail boleh laku daripada yang saya jangkakan, dan saya harap anda mempelajari sesuatu yang baharu serta saya menceritakan perjalanan saya.
Idea pertama saya ialah mengambil kod yang saya tulis dalam C, pecahkan menjadi beberapa bahagian, dan kemudian susun semula kepingan itu menggunakan Scratch. Sebagai contoh, gelung sementara dalam C mungkin menjadi ulangan sehingga blok dalam Gores:
Untuk membolehkan pengkompil C memahami kod, pertama sekali ia perlu menjana AST (pokok sintaks abstrak), yang merupakan perwakilan pokok bagi setiap simbol penting dalam kod sumber. Sebagai contoh, kurungan pembukaan, nama pembolehubah atau kata kunci pulangan mungkin setiap satunya ditukar kepada nod yang berbeza. Walau bagaimanapun, selepas melihat AST untuk program nombor Fibonacci yang mudah…
Baiklah, jadi itu di luar persoalan. Tetapi bagaimana jika daripada cuba menyusun semula kod sumber, kami turun satu langkah ke bawah: pemasangan? Untuk membolehkan atur cara berjalan, ia perlu disusun terlebih dahulu ke dalam perhimpunan. Pada komputer saya, itu x86-64asm. Memandangkan pemasangan tidak mempunyai sebarang struktur sarang, kelas atau pembolehubah yang rumit, cuba menghuraikan senarai arahan pemasangan seharusnya (secara teori) lebih mudah daripada cuba menghuraikan raksasa spageti AST, seperti yang di atas. Berikut ialah program Fibonacci yang sama tetapi dalam pemasangan x86.
Oh, abang. Baiklah, mungkin ia tidak begitu teruk. Berapakah jumlah arahan yang ada?
Syukurlah, x86 bukan satu-satunya bahasa himpunan di luar sana. Sebagai sebahagian daripada kelas kolej, saya belajar tentang MIPS, sejenis bahasa himpunan (terlalu memudahkan) yang digunakan dalam beberapa konsol permainan video dan superkomputer dari tahun 90-an hingga awal 2000-an, yang masih digunakan sehingga kini. Beralih daripada x86 kepada MIPS membawa kiraan bawah arahan daripada *tidak diketahui *kepada sekitar 50.
Menggunakan versi 32-bit MIPS, kod pemasangan ini kemudiannya boleh ditukar kepada kod mesin, di mana setiap arahan ditukar kepada integer 32-bit yang boleh difahami oleh pemproses, berdasarkan garis panduan yang ditetapkan oleh seni bina pemproses. Terdapat sebuah buku tentang seni bina set arahan MIPS tersedia dalam talian, jadi jika saya mengambil kod mesin, dan kemudian meniru dengan tepat apa yang akan dilakukan oleh pemproses MIPS, maka saya sepatutnya dapat menjalankan kod C saya dalam Scratch!
Sekarang itu tidak dapat dielakkan, kita boleh mulakan.
Nah, sudah ada masalah. Biasanya jika anda mempunyai integer dan anda ingin mengeluarkan satu siri bit daripadanya, anda mengira num & mask, dengan mask ialah integer di mana setiap bit penting ialah 1, dan setiap bit tidak penting ialah 0.
001000 01001 01000 1111111111111100 & 000000 00000 00000 1111111111111111 -------------------------------------- 000000 0000 000000 1111111111111100
Masalahnya? Tiada & pengendali dalam Scratch.
Sekarang, saya *boleh *hanya menelusuri kedua-dua nombor sedikit demi sedikit dan menyemak setiap satu daripada empat kemungkinan gabungan dua bit, tetapi itu akan membazir perlahan; lagipun, ini perlu dilakukan beberapa kali untuk *setiap *arahan. Sebaliknya, saya membuat rancangan yang lebih baik.
Mula-mula, saya menulis skrip Python pantas untuk mengira x & y bagi setiap x dan setiap y antara 0 dan 255.
for x in range(256):
for y in range(256):
print(x & y)
0 (0 & 0 == 0)
0 (0 & 1 == 0)
0 (0 & 2 == 0)
...
0 (0 & 255 == 0)
0 (1 & 0 == 0)
1 (1 & 1 == 1)
0 (1 & 2 == 0)
...
254 (255 & 254 == 254)
255 (255 & 255 == 255)
Sekarang, sebagai contoh, untuk mengira x & y bagi dua integer 32-bit, kita boleh melakukan perkara berikut:
Split x and y into four 8-bit integers (or bytes).
Check what first_byte_in_x & first_byte_in_y is by looking in the table generated from the Python script.
Similarly, look up what second_byte_in_x & second_byte_in_y is, and the third bytes, and the fourth bytes.
Take the results of each of these calculations, and put them together to get the result of x & y .
However, once a MIPS instruction has been cut up into four bytes, we’ll only & the bytes we need. For example, if we only need data from the first byte, we won’t even look at the bottom three. But how do we know which bytes we need? Based on the opcode (i.e. the “type”) of an instruction, MIPS will try to split up the bits of an instruction in one of three ways.
Putting everything together, below is the Scratch code to extract opcode, $rs, $rt, $rd, shamt, funct, and immediate for any instruction.
So, how much memory should our processor actually have? And how should we store it? Well, minimum, MIPS processors have 31 general-purpose registers, and one $zero register that is meant to store the number 0 at all times. A register is a location in memory that a processor can access quickly. We can represent these 32 registers as a list with 32 items in Scratch. As for the rest of the memory, simulating a processor moving chunks of data in and out of its cache in Scratch would be pretty pointless and would actually slow things down, rather than speed them up. So instead, the physical memory will be represented as five lists containing 131,072 elements each, where each element will be a 32-bit integer, giving us about 2.6MB of memory. A contiguous block of memory like these lists is usually called a “page”, and the size of the data that the instruction set works with (in this case 32 bits) is usually called a “word”.
So, how do we get machine code in here? We can’t just import a file into Scratch. But we *can *import text! So, I wrote a program in C to take a binary executable file, and convert every 32 bytes of the file into an integer. C, by default, was reading each byte in little-endian, so I had to introduce a function to flip the endianness. Then, I can save the machine code of a program as a text file (a list of integers), and then import it into my proc:memory:program variable.
#include <stdio.h>
unsigned int flip_endian(unsigned int value) {
return ((value >> 24) & 0xff) | ((value >> 8) & 0xff00) | ((value << 8) & 0xff0000) | ((value << 24) & 0xff000000);
}
int main(int argc, char* argv[]) {
if (argc != 3 && argc != 2) {
printf("Usage: %s <input file> <output file?>\n", argv[0]);
return 1;
}
FILE* in = fopen(argv[1], "r");
if (!in) {
perror("fopen");
return 1;
}
unsigned int value;
FILE* out = argc == 3 ? fopen(argv[2], "w") : stdout;
if (!out) {
perror("fopen");
return 1;
}
while (fread(&value, sizeof(value), 1, in) == 1) {
fprintf(out, "%u\n", flip_endian(value));
}
fclose(in);
if (out != stdout) {
fclose(out);
}
return 0;
}
Okay, so now that we can import the data into Scratch, we can just set the program counter (the integer keeping track of the current instruction) to the top of the list, and start executing instructions, right?
Wrong.
I didn’t realize this going into this project, but the first several bytes of an executable file *aren’t *instructions, but a header identifying what type of executable file it is. On Windows, it’ll usually be the PE, or Portable Executable, format, and on UNIX-based systems (the version we’ll be using) it’ll be the ELF format. So, how do we actually know where the code starts? On Linux, we can use the builtin readelf utility to actually see what’s in the ELF header, and the Linux Foundation has a page detailing the ELF header standard. So, we can use the LF page to figure out which bytes mean what, and the readelf command to “check our work”.
$ readelf -h fibonacci ELF Header: Magic: 7f 45 4c 46 01 02 01 00 00 00 00 00 00 00 00 00 Class: ELF32 Data: 2's complement, big endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: EXEC (Executable file) Machine: MIPS R3000 Version: 0x1 Entry point address: 0x4012cc Start of program headers: 52 (bytes into file) Start of section headers: 7596 (bytes into file) Flags: 0x1001, noreorder, o32, mips1 Size of this header: 52 (bytes) Size of program headers: 32 (bytes) Number of program headers: 5 Size of section headers: 40 (bytes) Number of section headers: 14 Section header string table index: 13
Now, there’s a lot of really interesting stuff here, but to save some time, the *really *important data here (besides the entry point, of course) are the section headers. Oversimplifying greatly, in order for our program to run correctly, we need to take certain chunks of the file and place them in certain parts of memory so our code can access them.
Using the readelf utility, we can actually see all of the sections in the file:
$ readelf -S fibonacci There are 14 section headers, starting at offset 0x1dac: Section Headers: [Nr] Name Type Addr Off Size ES Flg Lk Inf Al [ 0] NULL 00000000 000000 000000 00 0 0 0 [ 1] .MIPS.abiflags MIPS_ABIFLAGS 004000d8 0000d8 000018 18 A 0 0 8 [ 2] .reginfo MIPS_REGINFO 004000f0 0000f0 000018 18 A 0 0 4 [ 3] .note.gnu.build-i NOTE 00400108 000108 000024 00 A 0 0 4 [ 4] .text PROGBITS 00400130 000130 001200 00 AX 0 0 16 [ 5] .rodata PROGBITS 00401330 001330 000020 00 A 0 0 16 [ 6] .bss NOBITS 00411350 001350 000010 00 WA 0 0 16 [ 7] .comment PROGBITS 00000000 001350 000029 01 MS 0 0 1 [ 8] .pdr PROGBITS 00000000 00137c 000440 00 0 0 4 [ 9] .gnu.attributes GNU_ATTRIBUTES 00000000 0017bc 000010 00 0 0 1 [10] .mdebug.abi32 PROGBITS 00000000 0017cc 000000 00 0 0 1 [11] .symtab SYMTAB 00000000 0017cc 000380 10 12 14 4 [12] .strtab STRTAB 00000000 001b4c 0001db 00 0 0 1 [13] .shstrtab STRTAB 00000000 001d27 000085 00 0 0 1 Key to Flags: W (write), A (alloc), X (execute), M (merge), S (strings), I (info), L (link order), O (extra OS processing required), G (group), T (TLS), C (compressed), x (unknown), o (OS specific), E (exclude), p (processor specific)
Going through all the details of the ELF format could be its own multi-part write-up, but using the Linux Foundation page on section headers, I was able to decipher the section header bytes of the program, and copy all the important bytes from the proc:memory:program variable to the correct places in memory, by checking whether or not the section header had the ALLOCATE flag set.
Fast-forwarding about a week to the point where all of the important instructions have been implemented, let’s take a look at the steps the processor (or really, any processor) needs to take in order to understand just one instruction, using 0x8D02002A (2365718570) as an example.
The first step is called **INSTRUCTION FETCH. **The current instruction is retrieved from the address stored in the proc:program_counter variable.
The next step is INSTRUCTION DECODE, where the instruction is decoded into its separate parts (see Step 1).
Finally, we reach EXECUTE, which, in my Scratch processor, is pretty much just a big if statement.
In this case, the INSTRUCTION DECODE step revealed that the opcode is 35, which means 0x8D02002A is a lw (load word) instruction. Therefore, based off the values in proc:instr:rs, proc:instr:rt, and proc:instr:immediate, the instruction 0x8D02002A actually means lw $2, 0x2a($8) , or in other words, lw $v0, 42($t0).
And here is the code that handles the lw instruction:
Okay, home stretch. Now, we just need to be able to do the bare minimum and create a “Hello, World” program in C, and run it in Scratch, and the last two weeks of my life will have been validated.
So, will this work?
#include <stdio.h>
int main() {
printf("Hello, world!");
return 0;
}
Three changes. First of all, the MIPS linker uses start to find the entry point of the program, much the same way you use main in C, or "main__" in Python. So, that’s an easy fix.
#include <stdio.h>
int __start() {
printf("Hello, world!");
return 0;
}
Next, we need some way to actually see this output in Scratch. We *could *make some intricate array of text sprites, but the simpler solution is just to use a list.
Finally, we can’t use stdio.h.
Yeah, basically, implementing floating point registers and multiprocessor instructions would have been more trouble than it was worth, so I skipped it, but the standard library kind of expects all that to be there. So, we need to make printf ourselves.
Putting the complications of variadic arguments and text formatting aside, how can you actually print a string using MIPS? The TL;DR is you put the address of the string in a certain register, and then a special “print string” value in another register, and then execute the syscall (“system call”) instruction, and let the OS/CPU handle the rest.
The exact special values and registers to use are implementation-dependent, and can be implemented pretty much any way you see fit, but I chose to replicate MARS’ (a very popular MIPS simulator) implementation. With MARS, the address of the string goes in $a0, and the value 4 goes in $v0 to say “hey, I want to print a string!”
And with C, we can use a feature called “inline assembly” to inject assembly code directly into our compiled output. Putting it all together we get this:
#define puts _puts
void _puts(const char *s) {
__asm__(
"li $v0, 4\n"
"syscall\n"
:
: "a"(s)
);
}
int __start() {
puts("Hello, World!\n");
return 0;
}
And when we run it, we get this:
You can view the final product here: https://scratch.mit.edu/projects/1000840481/.
I wanted to keep this read under 15 minutes, so I had to skip over **a lot **of details. Some parts of the Scratch code had to be cut out of the screenshots for simplicity’s sake and I ran into a lot of silly and not-so-silly mistakes. If you’re curious how I was able to get Connect Four working (with minimax and alpha-beta pruning), the source code is on my Github. Here’s a quick list of some of the other problems I ran into in development:
* The fact that my computer is little-endian, but MIPS is big-endian caused more issues than I'd like to admit * The `mult` instruction in MIPS is 32-bit multiplication, and multiplying two 32-bit integers can result in a 64-bit integer. Javascript (and as a result, Scratch) is incapable of storing a 64-bit integer without losing precision. * The `u` in the `addu` instruction and the `u` in the `sltu` instruction both stand for "unsigned", but mean completely different things. * As you may have noticed, functions in Scratch don't have return values. This was quite annoying. * Any branch instruction (like "jump", "jump register", "branch on equals") in MIPS will also execute the instruction immediately after it, **regardless** of if the branch was taken or not. So, instead of updating the program count directly, the next address needs to be put in the "branch delay slot" and the program counter should only be updated after the *next* instruction. * Lists in Scratch are one-indexed. * All of a sudden, Scratch stopped letting me save the project to the cloud. It took awhile before I realized that lists filled with over 100,000 items wasn't something Scratch's servers were particularly excited to store. * I had to design my own `malloc` in C, which was fun, but also very difficult to debug in Scratch. * When I tried making syscalls that asked the user for input, all of the letters ended up capitalized. It turns out that in Scratch a lowercase `"a"` and a capital `"A"` are considered equal. I thought this was an unsolvable problem for awhile, before I realized that the names of sprites' costumes in Scratch are actually case-sensitive. So every time I try to convert a character to its ASCII value, I tell the processor sprite to switch to, for example, the `"a"` costume or the `"A"` costume, and then retrieve the costume number. * I made another syscall to print emojis to the `stdout`, but some emojis are considered two characters long and other emojis are considered one character long. * Compiling any code that calls `malloc` with -O1 crashes the CPU. I still have no idea why this is the case. * Endianness is really hard to get right. I know I said this in the beginning of the list, but it's worth repeating.
With all that said, I’m really happy with the way this project turned out. If you found this interesting, please check out sharc, my graphics engine built completely in Typescript: https://www.sharcjs.org. Because clearly, if there’s one thing I know how to make, it’s questionable decisions.
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