SvarDOS
Make sure local_chars does not have to be pinned to a specific memory location
Code tries to pin it to 0600h, which it cannot. Code also mentiones that this is located at 061Bh in DOS 4. Should be checked by inspecting MS-DOS 2 and 4 source code / memory map.
In msbio.tls there is a variable at 61Bh. I presume this is the one we're looking for.
Variable definition: https://hg.pushbx.org/ecm/msdos4/file/e58e166f626e/src/BIOS/MSBDATA.INC#l668
Uses in this file at the indicated spot and several more: https://hg.pushbx.org/ecm/msdos4/file/e58e166f626e/src/BIOS/MSCON.ASM#l70
Variable definition and some uses: https://hg.pushbx.org/ecm/edrdos/file/bda60c09a545/drbio/init.asm#l660
More uses: https://hg.pushbx.org/ecm/edrdos/file/bda60c09a545/drbio/console.asm#l134
In EDR-DOS there are two bytes used as content then flag. In MS-DOS the one single byte is used as both.