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IBM Monochrome Attributes

The original IBM Monochrome Display Adapter (MDA) had some idiosyncrasies that PCjs didn’t get quite right until recently. It all started when I was looking into this GitHub Issue regarding the best color choice for simulating the green phosphor of an IBM 5151 monochrome monitor.

I started with this screenshot of an actual IBM 5151 monitor in my garage:

The text was generated using a small ROM BASIC program that I wrote for this exercise:

10 DEFINT A-Z:DEF SEG=&HB000
20 CLS:V=0:OUT &H3B8,&H9
25 REM FOR EVERY ATTRIBUTE A, DISPLAY THE ATTRIBUTE IN HEX
30 FOR A=0 TO 255
35 H=A\16:GOSUB 60:H=A MOD 16:GOSUB 60:H=16:GOSUB 60
40 IF A MOD 16=15 THEN V=V+64:PRINT
45 NEXT A
50 REM OUT &H3B8,&H29 TO RE-ENABLE BLINKING
55 END
60 REM DISPLAY HEX DIGIT H AT VIDEO ADDRESS V WITH ATTRIBUTE A
65 IF H<10 THEN H=H+&H30
70 IF H<16 THEN H=H+&H41-10
75 IF H=16 THEN H=&H20
80 POKE V,H:POKE V+1,A:V=V+2
85 RETURN

I eventually settled on #09CC50 for PCjs’ default monochromatic green, replacing my original choice of #7FC07F. However, you’re free to override the default color of any machine by adding a fontColor parameter to the machine’s URL. For example:

https://www.pcjs.org/?fontColor=3350CC

will configure the IBM PC on the PCjs home page with a blue font. Avoid colors with an R, G, or B component set to the maximum value (FF), because otherwise there will be no difference between normal and bright intensities.

Next, I added a special PCjs Debugger command (“ev b000:0000”) to display every possible attribute on a monochrome screen in the same format, and this is what I saw:

Obviously, that didn’t match reality, but the fixes were fairly straightforward, and you can see live results in the machine below.

It’s interesting that IBM neutered the MDA in several unusual ways. For instance, there aren’t any attribute combinations that yield underlining and reverse video. There doesn’t seem to be any technical reason for the omission, since underlining is a foreground attribute and reverse video is a background attribute. And there aren’t any combinations that yield reverse video with bright foreground characters either.

Other non-IBM monochrome adapters may have included some or all of those combinations, but that’s an investigation for another day. For now, enjoy PCjs’ improved IBM compatibility.

NOTE: To change the adapter’s blink setting, you can either use PCjs Debugger commands to enable (o 3b8 29) or disable (o 3b8 9) blinking, or you can use a BASIC command, such as:

OUT &H3B8,&H29

[GitHub Source]

Jeff Parsons
Nov 15, 2018