Incomplete/broken mc6845 interlaced mode

[mgarlanger]

MAME version

0.259

System information

Macbook Pro 16" M1. Mac OS 14.0, bgfx.

INI configuration details

<none>

Emulated system/software

h19 with tlb slot device gp19 selected.

Incorrect behaviour

Only half the screen (25 lines) is shown when in 50 line mode.

Expected behaviour

Should see all 50 lines.

Steps to reproduce

1. start mame
2. select Heathkit h19 system
3. bring up menu to select slot device - tlb, select "gp19".
4. reset the system, it will be in 80x25 character mode.
5. Put it in 132x50 character mode with the following ESC sequence

ESC : 2 ; 4 f

6. Either start typing and filling the screen or bring up the system settings by pressing Ctrl-ESC
7. Count the screen lines.

Additional details

Screen shot of it with the problem:

With a partial fix from the following PR - https://github.com/mamedev/mame/pull/11504 it looks like this (which matches what my real system looks like):

[simzy39]

These also use the mc6845

-ACT Apricot PC/Xi https://github.com/mamedev/mame/blob/master/src/mame/act/apricot.cpp

-Victor 9000 https://github.com/mamedev/mame/blob/master/src/mame/act/victor9k.cpp

-Videx UltraTerm 80/132/160-column card https://github.com/mamedev/mame/blob/master/src/devices/bus/a2bus/a2ultraterm.cpp

-Amstrad CPC https://github.com/mamedev/mame/blob/master/src/mame/amstrad/amstrad.cpp

-Monochrome Display Adapter (MDA) section https://github.com/mamedev/mame/blob/master/src/devices/bus/isa/mda.cpp