a from-the-bootloader x86 os to try out some ideas in systems theory but mostly to tide me over this running injury. already did breads n plants n wanted to get in early on the next big quarantine hobby :'-)

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i'm on an m1 running macos 13.13.1. we'll need gcc--i built and installed 13.2.0. as stated in gcc's docs, build outside the source directory. there are also some build and runtime dependencies to be wary of.

mkdir gcc-13.2.0.host.build
cd gcc-13.2.0.host.build
../gcc-13.2.0/configure \
--prefix=/usr/local/gcc-13.2.0 \
--program-suffix=-13.2 \
--enable-checking=release \
--with-sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk \
--with-gmp-include=... \
--with-gmp-lib=... \
--with-mpc=... \
--with-mpfr-include=... \
--with-mpfr-lib=...

make -j8
sudo make install-strip

next, set up the environment:

# use gcc we just installed
export CC=/usr/local/gcc-13.2.0/bin/gcc-13.2
export CXX=/usr/local/gcc-13.2.0/bin/g++-13.2
export CPP=/usr/local/gcc-13.2.0/bin/cpp-13.2
export LD=/usr/local/gcc-13.2.0/bin/gcc-13.2

# home for our cross toolchain so that its isolated from the hosts'
export PREFIX=/usr/local/cross
export PATH="$PREFIX/bin:$PATH"

export TARGET=i686-elf

build cross binutils. do this before building cross gcc, which will link target libraries. i had to install texinfo first.

mkdir binutils-2.41.cross.build
cd binutils-2.41.cross.build
../binutils-2.41/configure \
--target=$TARGET \
--prefix="$PREFIX" \
--with-sysroot \
--disable-nls \
--disable-werror \
--with-gmp-include=... \
--with-gmp-lib=... \
--with-mpfr-include=... \
--with-mpfr-lib=...

make -j8
sudo make install

finally, our cross compiler::

mkdir gcc-13.2.0.cross.build
cd gcc-13.2.0.cross.build
../gcc-13.2.0/configure \
--prefix="$PREFIX" \
--target=$TARGET \
--disable-nls \
--enable-languages=c,c++ \
--without-headers \
--with-sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk \
--with-gmp-include=... \
--with-gmp-lib=... \
--with-mpc=... \
--with-mpfr-include=... \
--with-mpfr-lib=...

make -j8 all-gcc
make -j8 all-target-libgcc
sudo make install-gcc
sudo make install-target-libgcc

currently, the only supported boot medium is a disk image. host/scripts/mkimg (invoked as part of make) will create an 80mb single-partition fat32 image for this purpose.

debugger debugging

i wasn't able to get lldb to load the kernel--although i made only a pitiful effort to do so--and saw mention of 32 bit support being dropped in catalina so i went back to gdb. the version (8.0.1) of gdb i had brew install'ed a long while ago didn't support 32 bit ELFs so i reinstalled it. i think there used to be a --with-all-targets install option but it looks like that is default enabled now, so brew install gdb is all that's needed to get a version (e.g. 9.2) that can load our kernel. to check supported architectures, try:

(gdb) show configuration
(gdb) set debug arch 1
(gdb) set architecture <tab> # should show lots of stuff (200 for us)
(gdb) set gnutarget <tab> # should show lots of stuff (200 for us)

also, it looks like i8086 doesn't support real mode assembly, e.g. set architecture i8086; break *0x7c00; layout asm still shows 32 bit code :-(, but our good friends at so have come up with a fix.

some other useful tidbits:

for some reason, we have to symbol-file before file (possible gdb bug):

make gdb # will output symbols in build/bin/beans.sym
(gdb) symbol-file ./build/bin/beans.sym
(gdb) file ./build/bin/beans
(gdb) target remote :1234
(gdb) break src/kernel/main.c:<somewhere useful>
(gdb) print *loopy
(gdb) x/4bx <whatever is in loopy->mod_start> # gdb can't examine this memory
(gdb) stepi # until in the module
(gdb) x/i $pc # should see our loopy jmps
...

there are some convenience scripts in host/scripts/gdb, e.g.

gdb -command host/scripts/gdb/boot
# breaks at start of partition probing, in relocated mbr (offset from 0x600)
gdb -command host/scripts/gdb/mbr