3 ms·

No, there was a demo that fit on a 1.44 MB floppy from the late 90s (and thus only needing to support functionality of the time) [1]. As stated on the page, the

by Veserv 3y ago

No, there was a demo that fit on a 1.44 MB floppy from the late 90s (and thus only needing to support functionality of the time) [1]. As stated on the page, the kernel itself occupied 45 KB at the time. A current version would need additional lines to support features ubiquitous in modern chips such as having multiple cores. To put a 2 KB OS into context, the average x86 instruction is going to be around 3-4 bytes, so that OS would need to be around 500 assembly instructions in total. [1] http://web.archive.org/web/20011106140711/http://www.qnx.com/demodisk/how.html http://web.archive.org/web/20011106140711/http://www.qnx.com...

MichaelZuo 3y ago
Thanks for the links but 45 KB is likely closer to 2 KB then 2K LOC. So I think the point about room for improvements still stands. A minimal OS that supports the features included in modern ISAs such as ARMv8 or x64 would be way more then 2k LOC, probably more then 20k LOC.
- Veserv 3y ago
  Actually, 45 KB is pretty close to what you might expect from 2 KLOC of C code. Your average line of C will compile down to around 3-5 instructions in ARMv8 and x86-64 at around 3-5 bytes per instruction that is ~10-20 bytes per line. Also, I was pointing out that 2 KB of executable code you originally suggested for QNX is a suspicious number and should set off warning bells and how to do the back of the envelope calculation to build the intuition for why it is suspicious.
  - MichaelZuo 3y ago
    Huh, I didn't realize the average line of C code worked out to 3-5 instructions.
    - brucehoult 3y ago
      It depends on the line! Many lines of C will translate to a single instruction. Arithmetic expressions generally need one instruction per C operator. Each array access might need one or two extra instructions. A function call will need one instruction to move each argument into place (unless the argument is the result of arithmetic or array/pointer dereference in which case that part is for free), plus one or two instructions to jump to the function being called.
  - brucehoult 3y ago
    I actually built it. It's a whooooole lot bigger than 45 KB. It's bigger than a 1.44 MB floppy. bruce@rip:~/riscv/egos/egos-2000/build/release$ size -t * text data bss dec hex filename 1552 8 3076 4636 121c cat.elf 2232 8 3076 5316 14c4 cd.elf 1440 1076 2048 4564 11d4 clock.elf 3692 2120 2104 7916 1eec crash1.elf 424 8 2048 2480 9b0 crash2.elf 4294256 2496 3400 4300152 419d78 earth.elf 224 8 2048 2280 8e8 echo.elf 7144 12 2380 9536 2540 grass.elf 1136 8 3076 4220 107c ls.elf 156 8 2048 2212 8a4 pwd.elf 2928 1076 3076 7080 1ba8 sys_dir.elf 7972 2120 2132 12224 2fc0 sys_file.elf 4340 24 3088 7452 1d1c sys_proc.elf 1736 8 2048 3792 ed0 sys_shell.elf 188 8 2048 2244 8c4 ult.elf 4329420 8988 37696 4376104 42c628 (TOTALS) I'm not sure how the heck earth.elf text section is 4 MB. Sure, it's got a ton of libgcc stuff (e.g. mul/dev emulation) and libc stuff (e.g. big printf stuff). But it gzips to 100k and objdump shows 23676 instructions, which is 94704 bytes. Hex dumping the file, earth.elf is full of 0s from 0x1a9d0 to 0x4ab000 which is 4785712 bytes. Changing the ISA in the makefile from RV32I to RV32IMAC reduces all the text sizes except earth.elf by 20% to 30%, as expected. text data bss dec hex filename 1224 8 3076 4308 10d4 cat.elf 1694 8 3076 4778 12aa cd.elf 868 1076 2048 3992 f98 clock.elf 2712 2120 2104 6936 1b18 crash1.elf 324 8 2048 2380 94c crash2.elf 4264910 2496 3400 4270806 412ad6 earth.elf 164 8 2048 2220 8ac echo.elf 5346 12 2380 7738 1e3a grass.elf 928 8 3076 4012 fac ls.elf 110 8 2048 2166 876 pwd.elf 2066 1076 3076 6218 184a sys_dir.elf 6074 2120 2132 10326 2856 sys_file.elf 3474 24 3088 6586 19ba sys_proc.elf 1350 8 2048 3406 d4e sys_shell.elf 146 8 2048 2202 89a ult.elf 4291390 8988 37696 4338074 42319a (TOTALS)
    - MichaelZuo 3y ago
      earth.elf might be a spurious file?
notorandit 3y ago
LOC is not instruction. I see it is basically C, so a single LOC (think about an "if" or any loop instruction) would require more than a bunch of MC instructions.

by Veserv 3y ago

MichaelZuo 3y ago

Thanks for the links but 45 KB is likely closer to 2 KB then 2K LOC. So I think the point about room for improvements still stands. A minimal OS that supports the features included in modern ISAs such as ARMv8 or x64 would be way more then 2k LOC, probably more then 20k LOC.

Veserv 3y ago
Actually, 45 KB is pretty close to what you might expect from 2 KLOC of C code. Your average line of C will compile down to around 3-5 instructions in ARMv8 and x86-64 at around 3-5 bytes per instruction that is ~10-20 bytes per line. Also, I was pointing out that 2 KB of executable code you originally suggested for QNX is a suspicious number and should set off warning bells and how to do the back of the envelope calculation to build the intuition for why it is suspicious.
- MichaelZuo 3y ago
  Huh, I didn't realize the average line of C code worked out to 3-5 instructions.
  - brucehoult 3y ago
    It depends on the line! Many lines of C will translate to a single instruction. Arithmetic expressions generally need one instruction per C operator. Each array access might need one or two extra instructions. A function call will need one instruction to move each argument into place (unless the argument is the result of arithmetic or array/pointer dereference in which case that part is for free), plus one or two instructions to jump to the function being called.
- brucehoult 3y ago
  I actually built it. It's a whooooole lot bigger than 45 KB. It's bigger than a 1.44 MB floppy. bruce@rip:~/riscv/egos/egos-2000/build/release$ size -t * text data bss dec hex filename 1552 8 3076 4636 121c cat.elf 2232 8 3076 5316 14c4 cd.elf 1440 1076 2048 4564 11d4 clock.elf 3692 2120 2104 7916 1eec crash1.elf 424 8 2048 2480 9b0 crash2.elf 4294256 2496 3400 4300152 419d78 earth.elf 224 8 2048 2280 8e8 echo.elf 7144 12 2380 9536 2540 grass.elf 1136 8 3076 4220 107c ls.elf 156 8 2048 2212 8a4 pwd.elf 2928 1076 3076 7080 1ba8 sys_dir.elf 7972 2120 2132 12224 2fc0 sys_file.elf 4340 24 3088 7452 1d1c sys_proc.elf 1736 8 2048 3792 ed0 sys_shell.elf 188 8 2048 2244 8c4 ult.elf 4329420 8988 37696 4376104 42c628 (TOTALS) I'm not sure how the heck earth.elf text section is 4 MB. Sure, it's got a ton of libgcc stuff (e.g. mul/dev emulation) and libc stuff (e.g. big printf stuff). But it gzips to 100k and objdump shows 23676 instructions, which is 94704 bytes. Hex dumping the file, earth.elf is full of 0s from 0x1a9d0 to 0x4ab000 which is 4785712 bytes. Changing the ISA in the makefile from RV32I to RV32IMAC reduces all the text sizes except earth.elf by 20% to 30%, as expected. text data bss dec hex filename 1224 8 3076 4308 10d4 cat.elf 1694 8 3076 4778 12aa cd.elf 868 1076 2048 3992 f98 clock.elf 2712 2120 2104 6936 1b18 crash1.elf 324 8 2048 2380 94c crash2.elf 4264910 2496 3400 4270806 412ad6 earth.elf 164 8 2048 2220 8ac echo.elf 5346 12 2380 7738 1e3a grass.elf 928 8 3076 4012 fac ls.elf 110 8 2048 2166 876 pwd.elf 2066 1076 3076 6218 184a sys_dir.elf 6074 2120 2132 10326 2856 sys_file.elf 3474 24 3088 6586 19ba sys_proc.elf 1350 8 2048 3406 d4e sys_shell.elf 146 8 2048 2202 89a ult.elf 4291390 8988 37696 4338074 42319a (TOTALS)
  - MichaelZuo 3y ago
    earth.elf might be a spurious file?

notorandit 3y ago

LOC is not instruction. I see it is basically C, so a single LOC (think about an "if" or any loop instruction) would require more than a bunch of MC instructions.