AGI definitely uses the video card font data for fonts. It does some weird stuff with them though, that I haven't been able to fully decipher. According to MSDOS reference material, there are 8x8 font data for characters 00 - 7Fh at INT 43h (memory location F000:FA6E) and 8x8 font data for characters 80h - FFh at INT 1Fh. When AGI puts characters to screen, it looks like it grabs the font data from INT 43h, regardless of the character value (which explains the garbled output, since >7Fh will be using data outside the range for this location). But during startup, when various DOS interrupt vectors are reassigned, AGI does reassign the vector for INT 1Fh (the extended character data) to the location where each character is output before being sent to the screen. I don't know yet why this is being done, or what effect it ends up having.
Regardless, original Sierra AGI does not display/support extended characters.
But NAGI can; I actually wrote a NAGI font editor ten years ago that you can use to edit/create your own NAGI fonts, including creating characters for all 255 byte codes. I'm attaching the install file for NAGI Font Editor if you want to give it a try; just open one of the *.nbf files included in NAGI to change the fonts however you want. I've also attached a 'spanish' version of the 16x16 font file I used for testing. (You can also get NFE from
www.sierrahelp.com -
http://www.sierrahelp.com/AGI/Files/Misc/NAGI_Font_Editor/NAGI_Font_Editor_(NFEd_101).zip)
Note that WinAGI can handle messages using extended codes, if you want to create a sample game to test the fonts in NAGI.
BUT, the rich text editor used for syntax highlighting reacts a bit goofy when extended codes are involved, so I'm adding a feature to turn off syntax highlighting in case anyone wants to use extended codes.
But just be aware that while you can use NAGI to display extended characters, AGI input is strictly limited to the regular characters; the text input/parsing routines and words encoding in the WORDS.TOK file were written to explicitly preclude the possibility of entering extended characters as input.