3 ms·

I may have misunderstood, but I'm pretty sure APE is not a "binary format" per se. It is a script that can be executed on any system. That script can then LOAD

I may have misunderstood, but I'm pretty sure APE is not a "binary format" per se. It is a script that can be executed on any system. That script can then LOAD a binary. IIRC the original needed to decode it from base64 before it could be loaded. So... it's an executable binary loader

rep_lodsb 3y ago
It's a script that starts with an EXE header ("MZ"). Having both EXE and ELF headers at the same location is obviously impossible, since they start with different bytes. A possible, although very limited way to have an actual binary program execute on different platforms would be to create a DOS .COM file (which has no header, just the raw machine code) with a valid ELF header. It would then also work on 32-bit Windows via its built-in DOS emulator, and presumably on 64-bit Windows with WSL2. The start bytes for a 32-bit ELF header decode as 16-bit x86 into: 7F 45 JG +45h 4C DEC SP 46 INC SI 01 01 ADD [BX+DI],AX 01 xx ADD ... The first instruction is a jump past the end of the ELF header, unfortunately it's conditional. But we have 9 reserved bytes to continue this code, which is enough to undo the effects of the DEC and ADD instructions and then jump to the same address. I've written a 138 byte "Hello world" that works on Linux, DOS and also CP/M-80 that way. It's possible to have the code that executes under Linux be a small (less than 2K bytes) loader program that creates 16-bit code and data segments and installs a handler for SIGSEGV. It can then jump into the same code that would run under 16-bit DOS, trapping every INT 21h and translating the most important syscalls into their Linux equivalents, kind of like a minimal version of Wine. I have a proof of concept for that, it only handles the "read", "write" and "exit" syscalls, which is enough to write something like rot13 or hexdump. With a lot more work, it could be possible to produce really non-trivial software that runs in such a restricted environment...
- vitiral 3y ago
  Don't tell jart this or she'll probably add it to cosmopolitan
actionfromafar 3y ago
Same same, but different… How can the syscalls work the same in linux, windows and n my macos?! It’s bananas
- vitiral 3y ago
  They don't. APE loads a different syscall table depending on the platform. Or something like that, I don't claim to fully understand the internals

rep_lodsb 3y ago

It's a script that starts with an EXE header ("MZ"). Having both EXE and ELF headers at the same location is obviously impossible, since they start with different bytes. A possible, although very limited way to have an actual binary program execute on different platforms would be to create a DOS .COM file (which has no header, just the raw machine code) with a valid ELF header. It would then also work on 32-bit Windows via its built-in DOS emulator, and presumably on 64-bit Windows with WSL2. The start bytes for a 32-bit ELF header decode as 16-bit x86 into: 7F 45 JG +45h 4C DEC SP 46 INC SI 01 01 ADD [BX+DI],AX 01 xx ADD ... The first instruction is a jump past the end of the ELF header, unfortunately it's conditional. But we have 9 reserved bytes to continue this code, which is enough to undo the effects of the DEC and ADD instructions and then jump to the same address. I've written a 138 byte "Hello world" that works on Linux, DOS and also CP/M-80 that way. It's possible to have the code that executes under Linux be a small (less than 2K bytes) loader program that creates 16-bit code and data segments and installs a handler for SIGSEGV. It can then jump into the same code that would run under 16-bit DOS, trapping every INT 21h and translating the most important syscalls into their Linux equivalents, kind of like a minimal version of Wine. I have a proof of concept for that, it only handles the "read", "write" and "exit" syscalls, which is enough to write something like rot13 or hexdump. With a lot more work, it could be possible to produce really non-trivial software that runs in such a restricted environment...

vitiral 3y ago
Don't tell jart this or she'll probably add it to cosmopolitan

actionfromafar 3y ago

Same same, but different… How can the syscalls work the same in linux, windows and n my macos?! It’s bananas

vitiral 3y ago
They don't. APE loads a different syscall table depending on the platform. Or something like that, I don't claim to fully understand the internals