New BOOT -bios option

[joncampbell123]

I just added initial code to support booting to a ROM BIOS image using the BOOT command.

Initial test results so far below.

IBM PC/AT mode (using ROM images from the PCem project):

• AMI 286 BIOS does not display anything, issues morse code to PC speaker indicating a problem
• AMI 386 and AMI 486 just make "buzzing" noises to the PC speaker, no progress
• IBM PC (5150) BIOS from 1982 fails to set up a stack, jumps to zeros, and then to some arbitrary code following the zeros. I don't think it's supposed to do that. It does poke at CGA registers at some point which suggests that getting it to work would have visible results when machine=cga.

PC-98 mode:

• PC-9821 ROM BIOS image spins in an endless loop, checking some I/O ports, writing an opcode (OUT DX,AL + JMP FFFF:0000) to RAM, loading DX=43Dh and AL=00h and then JMPing to that opcode. One I/O write is issued, and then jumps to FFFF:0000 to begin execution of the BIOS from the start again. According to the webtech undocumented PC-98 I/O port list, port 43Dh controls bank switching of PC-98 ROM BIOS. Writing 00h to that port is supposed to bring in the contents of ITF.ROM to F8000-FFFFF which apparently contains the actual reset vector and startup code.

DOSBox-X won't be able to support running N88 BASIC without the ability to run the PC-98 ROM BIOS, which is the reason for this addition.

A bonus is that booting ROM BIOS images (in either IBM or NEC mode) enables analysis of the boot process.

[J1mbo]

@joncampbell123 Thanks for this mod - exactly what I'm looking for. The BIOS file causes DosBox-X to apparently hang in my case, but how do you produce that diagnostic output?

[joncampbell123]

@J1mbo Run it with --debug for extra information.

You may also want to run with -break-start to make DOSBox-X break into the debugger before it executes the first instruction in the BIOS.