Hercules Graphics Card
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The Hercules Graphics Card (HGC) was a mid-1980s computer graphics controller which through its popularity became a de-facto display standard. It was common on IBM PC compatibles connected to a monochrome (green, amber or, less often, black-white) phosphor monitor. It supported one high resolution text mode and a single graphics mode.
Its monochrome text mode could display 80×25 text characters and was MDA compatible. As such, it rendered characters in a box of 9×14 pixels, of which 7×11 made out the character itself (the other pixels being used for space between character columns and lines). This amounted to markedly clearer text display than the CGA adapter (which the Hercules card competed against) could offer. The total theoretical resolution of this text mode was 720×350 pixels. This number is arrived at through multiplying the character width of 9 pixels by the number of text columns possible on screen (80) as well as multiplying the character height of 14 pixels by the number of text lines (25). In the MDA compatible text mode however, these pixels were not individually addressable.
The Hercules cards's single monochrome graphics mode simply made all pixels directly addressable. This however translated to a resolution of not 720×350 but only 720×348 pixels (at 1 bit per pixel) because, for technical reasons, two lines were not displayed.
Its provision of an MDA compatible high quality text mode in combination with a (for its time) high resolution graphics mode (as well as aggressive pricing) made the Hercules card extraordinarily popular in the early days of the PC. The existence of CGA emulation drivers/TSRs, which allowed Hercules users to run programs written for the CGA card's standard graphics modes (albeit only in monochrome, without actual colour), may also have been a contributing factor to its success, especially so because programming for the Hercules card's native graphics mode was somewhat hindered by the fact that there was neither any BIOS support nor standardization from IBM — after all, the HGC was a competing technology!
Long after its prime, the Hercules card continued to be popular for specialist applications, such as some debuggers and CAD programs, because it could be used to connect a secondary monitor alongside another (colour) graphic adaptor. Certain software detected the HGC and used the monochrome display for extra data display while running the application on the other screen — for example a CAD work area would be displayed on the main (non-Hercules driven) screen and a list of drawing commands would be shown on the HGC driven monochrome screen.
Technical specifications
The graphic page was hardwired to address '0b000h' — different from the address '0a000h' used by newer (colour) graphic adaptors. The fact that these addresses didn't conflict with each other made dual screen operation possible, simply through installation of a Hercules card next to (for instance) a VGA adapter.
Competing adapters
- IBM's Color Graphics Adapter
- IBM's Monochrome Display Adapter