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Two adapters, one desktop

A Hercules/CGA pair on an original IBM PC, with windows spanning both displays.

Try it yourself A few details
One machine, two monitors
Two period PC monitors side by side on a desk, a green monochrome one on the left and a colour one on the right, both showing parts of the same os8088 desktop.
The demonstrated Hercules/CGA hardware pair.
Displays
2
Workspace
1 desktop
Hardware demonstration
IBM PC

01 / Display configuration

Display configuration

os8088 extends one desktop across a physical Hercules/CGA pair. Either display can be primary; the secondary can be placed to its right or below it. The demonstrated setup uses original IBM PC hardware.

IBM documents the monochrome and colour adapter mappings separately in its technical references. The os8088 arrangement also accounts for the Hercules graphics aperture; it is not a generic two-card configuration.

02 / Spanning-window captures

Spanning-window captures

The Hercules monitor
Close-up of the green monochrome monitor: the os8088 menu bar across the top, a file listing in the middle, the dock along the bottom, and at the top right a window cut off at the edge of the screen.
Primary-display portion of the spanning Note Pad window.
The CGA monitor
Close-up of the colour monitor: the rest of the same Note Pad window, its text continuing mid-word, with an About box open in front of it.
The same window continued on CGA.

03 / Window and pointer behaviour

Window and pointer behaviour

Windows can straddle the display boundary and are clipped independently by each adapter. Menu bar, dock and drive icons remain on the primary display. Full-screen programs claim one display while the other retains the desktop.

The pointer transfers whole between displays and is constrained around gaps caused by differing heights. Extended mode is opt-in because detecting an adapter does not establish that its monitor is connected.

Desktop coordinates

Different geometries leave unused space

Hercules primary with CGA to the right 720 by 348 Hercules on the left, 640 by 200 CGA on the right. The lower right region is unused. The illustrated window is clipped to the actual displays. 12 1 / Hercules 720 x 348 / primary2 / CGA 640 x 200 / secondary
Logical pixel geometry, with an illustrated window clipped across both displays. Hatching marks space outside either screen; physical CRT proportions differ. Display geometry and extension rules, section 39.

04 / Supported pair

Supported pair

Two displays maximum, using Hercules plus physical CGA. VGA CGA emulation is not a substitute for the second adapter. The secondary has no separate dock or menu bar. EGA is not a supported primary for extended mode.

05 / Enable extended mode

Enable extended mode

Connect both monitors before choosing Display > Desktop > Extend. Select the primary and relative placement. make xt-multimon provides the corresponding 86Box setup.

make xt-multimon
Download os8088 Source What else changed