Weird elf program that could be ran in both MS-DOS and unix?

[clementttttttttt]

So I found this weird program in grub4dos, which is called "bootlace.com", that can be ran in both MS-DOS and unix, and google didn't help. I would like an explanation on how did it do that.

[bzt]

So I found this weird program in grub4dos, which is called "bootlace.com", that can be ran in both MS-DOS and unix, and google didn't help.

Have you tried github? bootlace source.

I would like an explanation on how did it do that.

Use the source Luke...!

The DOS .com executable has no header, it just starts executing the code in real mode from the first byte. ELF needs a header, which starts with 4 magic bytes. Those magic bytes interpreted as real mode code gives:

Code: Select all

	# ELF64 header backup here upto the end of file. Its length is 0x40.

	.byte	0x7F, 0x45, 0x4C, 0x46	# ELF magic number
					// 7F 45 = jg dos_entry_point
					// 4C = decw %sp
					// 46 = incw %si

So assuming CPU flags are set, that "jg" instruction will jump to the "dos_entry_point", while under Linux the ELF header is parsed and the code specified by the ELF e_entry point will be executed instead (_start_linux). If CPU flags aren't set for the conditional jump, then the first 16 bytes of the file will be executed in real-mode, which then would jump to the same dos_entry_point.

Cheers,
bzt