Graphics systems of NEC PC-98
History
Introduction
NEC PC-9800 series was initially developed for 16-bit fast computing and compatibility with NEC's early 8-bit personal computer lines, PC-8001 and PC-8801. Later models were also designed for compatibility with early models. Some models have special graphics systems, but they were failure in the market due to their price and incompatibility.
All proprietary PC98 graphics systems are put on their motherboard (or a daughter board with the motherboard). You cannot replace nor remove it.
GDC (Graphic Display Controller, μPD7220)
In 1981, NEC introduced the uPD7220, one of the first computer GPU. First, it was used in their small business computer "N5200 model 05" released in the same year. This machine is an all-in-one desktop computer and its display resolution is 640×475 pixels, but its architecture is similar to PC98. It was exported to the United States as the APC (Advanced Personal Computer). The uPD7220 was also used in DEC Rainbow, Tulip System-1 and Number Nine's video card for the IBM PC. See also NEC uPD7220 - Wikipedia (en).
The first PC-9801 contains two uPD7220A (run at 2.5MHz), the one and 8KB video RAM for text, the other and 96KB video RAM for graphics. It supports four screen modes, 640×200 & monochrome & 6 pages, 640×200 & 8 colors & 2 pages, 640×400 & mono & 3 pages, and 640×400 & 8 colors & 1 page. It can only display Latin alphabetic, numeric and katakana characters (ANK文字). If an optional kanji ROM board (PC-9801-01 or -10) is installed, it can display 3,000 or more kanji characters quickly.
In 1983, NEC shipped out the PC-9801F which contained twice video RAM for more graphic pages, JIS Level-1 character set Kanji font ROM and additional 4KB text RAM for double-byte characters.
GRCG (GC, Graphic Charger)
In 1985, NEC introduced a graphics accelerator chip specialized for PC98 that is called "Graphic Charger", also known as "GRCG". It supports accessing multiple graphic planes at the same time, makes faster filling a bitmap. PC-9801U2 (May 1985), PC-9801VM (July 1985) and later models support it.
PC-9801VM was the first model supports 8 (16) colors chosen from 4096 colors with 8+256KB video RAM, but 16 colors was supported by using an optional video RAM board (PC-9801-24). The PC-9801UV2 (May 1986) and later models supported it by default.
EGC (Enhanced Graphic Charger)
It was introduced with PC-9801VX in 1986. It has a backward-compatibility with GRCG, and supports parallel processing with GDC, raster operation, bit shifter and block transfering. Some models, released soon after PC-9801VX, didn't support it. (eg: PC-9801UV, UR, CV, PC-98DO and early PC-9801 laptops.)
The configuration of 2 GDCs + EGC, 640×400 pixels, 16 colors and 2 pages became the standard of PC98 graphics. They had been supported until the last model PC-9821Ra43 (2000). PC-9801UX (1987) and laters didn't have a real μPD7220A. It was integrated into two custom chipsets, though this model had not combined the EGC yet.
High-resolution mode & machine
The High-resolution mode (ハイレゾリューションモード), shortly called High-reso mode (ハイレゾモード) or High-reso machine (ハイレゾ機), has a 768KB conventional memory, 1120×750 resolution and different graphics system. Its BIOS and I/O are partly compatible with normal PC98. It was supported by some models, PC-98XA, XL, XL2 and PC-H98 series. They were expensive and mostly used for CAD systems. I've never heard the PC game used it.
This mode is partially supported on 98MATE A (PC-9821A*) series, by using an optional 32-bit local bus expansion board (PC-9821A-E02).
PC-98GS
Only this model has a special graphics system which is called "Extended Screen Graphics" (拡張画面グラフィック). It has 1 MB video RAM and supports up to 640×240 & 24-bit colors & 1 page, or 640×480 & 65536 colors & 1 page, or 640×480 & 256 colors from 16M colors & 2 pages. It has a video capturing and sound recording system, but the machine requires a special monitor (PC-98GS-C1).
Its sound system was released as a single C-bus expansion board (PC-9801-73). And, its cost-reduced version became "PC-9801-86", the most common sound board for PC98.
256 color mode (PEGC)
It was introduced with the 98MATE (PC-9821Ap, As, Ae) in 1993. It supports two screen modes, 640×400 pixels & 256 colors chosen from 24-bit colors & 2 pages, and 640×480 & 256 colors & 1 page. It was similar to PC-98GS's Extended Screen Graphics but not compatible. It was used in Windows, a few business softwares and PC98 DOS games, Blandia, Doom, Dungeon Master II, Mugen Houyou, Rance IV, War Craft, etc. Its official name is unknown or simply called "256 color mode" (256色モード), but it's sometimes called "PEGC" by PC98 geeks because its name appeared in the Windows display driver and its binary.
In this graphics system, there are two video RAM accessing mode, the planar mode (プレーンモード, plane mode) and the packed-pixel mode (パックトピクセルモード) . The planar mode is supported only on a few models, PC-9821Ap, As, Ae, Af, Ap2, As2, An, Ce, Ce2, Cs2, Np and Ne. These machines are popular among PC98 gamers because they can run almost 90's PC98 games, built-in '86 sound', fully-supported 256 color modes, slightly better performance on MS-DOS and Windows 3.1. It is also famous that many unmaintained machines often have broken because they use bad capacitors like a PSU in Sharp X68000.
Window Accelerator (Windows Accelerator)
In 1993, NEC developed the Window Accelerator (ウィンドウアクセラレータ; third parties called it Windows Accelerator) which was a proprietary expansion board (later a PCI card) employed an SVGA chip. It could display higher resolutions and more colors on Microsoft Windows and OS/2. They are very similar to IBM PC's graphics cards, except they don't use a VGA BIOS. The PC98 graphics works in the text mode of its boot-up sequence and MS-DOS. When using the Window Accelerator, it is disabled by its display driver. Some models use a through cable to output both video signals from one display connector, and two signals are switched by the display driver.
There were other proprietary graphics systems from third parties, such as Digital Arts' HyPER FRAME and IO-DATA's GA-1024A, neither I don't mention them. Few games supported, but some Alice Soft's adult games supported 256 color mode of GA-1024A.
Compatibility issues
Early PC98 games run quickly on later models
Because some early 9801 games depend on CPU cycles, they run quickly on later models. Even CPU's clock speed is the same, its clock cycles per instruction and wait cycles of I/O are different (eg: 8086 8 MHz and V30 8 MHz) . Sometimes it's too fast to play an action game.
A bug in real EGC
The original EGC chip (marked as D65101S017) has a bug that one of the register operations doesn't work correctly in a specific case. NEC's mid-9801 machines (eg: PC-9801VX, RA, EX) has this issue. Later models and Epson's clones were fixed. Its difference may cause a compatibility issue. (But, not many PC98 games depend on EGC)
GDC run at 5 MHz is not supported in some DOS games
The PC-9801DA and laters can run the GDC at 5 MHz, but it causes a problem in some DOS games (eg: Puyo Puyo and Touhou 1-3). You need change the Software Dip Switches to run the GDC at 2.5 MHz. See System Setup Menu [NEC PC98].
30-line text mode cannot display correctly on some monitors
The PC98 has 8 or 12 KB of video RAM for the text screen. One or two bytes of video RAM are used for each character, and the memory is addressed between A0000h and A1FFFh (the attribute is placed at different location, starting at A2000h). Technically, the video RAM can store the text up to 4,096 characters, and the display controller can offer extra resolutions . However, the wider resolution decreases the refresh rate and causes flicker on a normal PC98 monitor. The PC-9821 models supports 31 kHz horizontal frequency (the PC98 generally uses 24 kHz horizontal frequency), used by some applications to set the 30-line text screen (≓ VGA 640×480 resolution). It requires a multisync monitor.
Standard 256-color display driver comes with Windows 3.x does not work
The standard 256-color driver bundled with retail versions of Windows 3.0B and 3.1 does not work on most PC-9821 machines because it requires the planar mode of PEGC. As mentioned above, this mode was supported only on few models. Display drivers for other machines came with their pre-installed version of Windows. At present, most of them are lost so it's difficult to find them.
DOS screen is smaller than a monitor screen on laptops with SVGA LCD
Because PC98 laptops don't support display scaling, the DOS screen become smaller and inserted black margins on SVGA or larger LCDs. The PC-9821Nx/C7 or PC-9821Na13/C10 is the most suitable for PC98 DOS games. The only one problem exists, they have YMF288 + CS4231 (FM & PCM sound chip) instead of the traditional YM2608 + PCM buffer. It sometimes causes compatibility issues, especially at playing PCM sound.
640x200, 40-column text and monochrome are no longer supported on late PC-9821 models.
PC-98s released in mid-1995 and later don't support 640x200 graphic screen, 40-column text and monochrome mode. If you want to test this, put these commands into N88-BASIC, "SCREEN 0" (switch the graphic screen to 640×200 & color), "SCREEN 2" (switch to 640×400 & mono) and "WIDTH 40" (switch the text screen to 40-column mode). If it is not supported, it cannot display correctly or returns an error. Late-1990s machines don't have a built-in BASIC interpreter.
Connecting a monitor
Most machines have a DA-15 connector which uses the same analog video signal format with VGA. It can be converted to DE-15 (VGA) connector with a pin adapter (Sanwa-supply AD-D15NE). However, its video mode is operated by 24 kHz horizontal frequency that is not supported by most modern PC monitors. If you use PC-9821 or later models (include PC-9801B*, except the first model S1, S2), you can switch its horizontal frequency to 31 kHz that is commonly used on VGA. To switch the horizontal frequency to 31 kHz, hold-down GRPH and "2" key (not ten-key) and turn on the PC. To switch back to 24 kHz, hold-down GRPH + 1 and turn on the PC. If you want to know other commands, see Special boot-up key commands [NEC PC98]. (Note 1: Some monitors cannot be centered even if you try to correct it manually. Note 2 : In 256 color mode, the horizontal frequency is fixed to 24 kHz.)
See also
External links
- NEC PC-9801 (Platform) - Giant Bomb - More general information for PC gamers. (in English)
- PC98 MS-DOS and Hardware Programming (web archived, in Japanese)
- EGCM055.LZH - The Public Domain Manual of Enhanced Graphic Charger Ver.0.55 (in Japanese)
- Switcher circuit at video output line in PC-9821 (in Japanese)
- List of all PCI graphics cards for PC-9821 which were available from NEC and third parties (in Japanese)
- AT machines video cards working on PC-98 (Win2000) (in Japanese)
- GeForce 8600GT works on PC-9821 with VGA BIOS modification (in Japanese)