PC-9821 Cheat Sheet

Set Horizontal Frequency

While booting press the “GRAPH” key and:

  • “1” for 24 kHz
  • “2” for 31 kHz

Note, not all hardware supports this.

DIP Switches

Bios DIPs

The PC-9821 uses soft dips. To access, turn on the power or press the reset button and then press the “HELP” key. To reset software dips press RESET + [GRPH] + [SHIFT].

The following DIP settings were translated at retrostuff.org and I have copied them here in the case that the site disappears one day.

For most DOS games use:

  • Switch 2 settings: 
    • GDC Clock: 2.5MHz
  • Switch 3 settings:
    • Floppy Disk Mode: Automatic (640K)

Keyboard Actions

Home CLRFactory settings
EscBack to main menu

DIP Switch 1 Settings

Plasma Display– Enabled
– Disabled
Hint: Usually disabled
Floppy Disk Sequence– Internal #3, #4 | External #1, #2 (prioritize ext FDD)
– Internal #1, #2 | External #3, #4 (prioritize int FDD)
RS-232C Transmission Mode– BCI Sync (use internal clock for transmission, use modem clock for reception)
– ST2 Sync (use modem clock for transmission and reception)
– Sync Clock (use clock signal created from received data for reception, use internal clock for transmission)
– Asynchronous (use internal clock for transmission and reception
Hard Disk Sector Size– 512 bytes
– 256 bytes
Graphics Mode– Enhanced (16 out of 4096 colors)
– Standard (8 out of 8 colors)

DIP Switch 2 Settings

Terminal Mode Specification– Terminal Mode (Start BASIC in terminal mode)
– BASIC Mode (Start BASIC in BASIC mode)
Lines of Text per Screen– 25 lines/screen (Display 25 lines of text per screen)
– 20 lines/screen (Display 20 lines of text per screen)
Memory Switch– Keep (Keep the contents of memory switch at startup)
– Initialize (Initialize the contents of memory switch at startup)
Internal Hard Disk– Disabled (Disable internal hard disk)
– Enabled (Enable internal hard disk)
GDC Clock– 5MHz (Operate GDC with 5MHz clock)
– 2.5Mhz (Operate GDC with 2.5MHz clock)

DIP Switch 3 Settings

Floppy Disk Mode– 640K (Use internal FDD with 640Kbyte fixed)
– 1M (Use internal FDD with 1Mbyte fixed)
– Automatic (640K) (Automatic recognition of 640K/1M. Use 640K when starting another device)
– Automatic (1M) (Automatic recognition of 640K/1M. Use 1M when starting another device)
Floppy Motor Control– Disable (Do not control floppy disk motor)
– Enable (Stop floppy disk motor if no access for 15 seconds)
DMA Clock– High Speed
– Compatible
Internal RAM Hole– Disabled (Disable memory hold at 512K-640K)
– Enabled (Enable memory hold at 512K-640K)
CPU Mode– High (Start in High mode)
– Low (Start in Low mode)

Operating Environment Settings

16MB Memory Hole– Disabled (The system does not use memory hold at 15M-16M)
– Enabled (The system uses memory hold at 15M-16M)
Hint: When using Windows 3.1 please set to “enabled”
Speaker Volume– High
– Medium
– Low
– Off
Hint: Set the volume of the internal buzzer

Hard DIPs

Apart from this, there are two switches in the cover on the front of the main unit. Each one is as follows. The pink highlight is the default.

Switching CPU Operating Speed (left side)

DisplayPower LampOperating Speed
Hgreeni486DX2 66MHz
Myellowi486SX equivalent to 16MHz
LredV30 equivalent to 8MHz


  • In V30 emulation mode, all memory larger than 640KB is ignored and fixed at 640KB. Note that software and expansion boards that require V30 may not work properly because they are not 100% completely compatible.
  • Also, if the operating mode is set to high resolution mode, V30 emulation mode can not be selected regardless of the presence of the 98 high resolution board.

Switching Operation Mode (right side)

DisplayAction Mode
NNormal mode
HHigh res mode (selectable when equipped with 98 high res board)


  • If the high resolution mode is set to the high resolution side without the 98 high resolution board, problems will occur in switching the operating speed, so be sure to set it to the normal side.

Window Accelerator Board A (PC-9821A-E01)

Switch NumberFunctionOnOff
1CRT display operation methodNon-interlacedInterlaced
2Not used (always on)
3Not used (always on)
4Not used (always on)
5CPU address space settingON, ON: F00000h to F0FFFFh, F80000h to F80FFFh
ON, OFF: F20000h to F2FFFFh, F88000h to F88FFFh
OFF, ON: F40000h to F4FFFFh, F90000h to F90FFFh
OFF, OFF: F60000h to F6FFFFh, F98000h to F98Fha
7Not used (always on)
8Not used (always on)

Open Source Scan Converter (OSSC)

Optimal Timings

Console / modeDots per scanline / no. scanlines / @dotclkOSSC base modeModified parameters
PC-98 640×400 @56.4Hz848 / 440 /  @21.0526MHz640×400H.samplerate=848

More PC-98 OSSC Links:

Write a Floppy from an Image on Real Hardware



  1. Convert your floppy disk image (.fdi, .hdm, etc) to Mahalito format using Virtual Floppy Image Converter.
  2. Transfer the Mahalito formatted image to your PC-98
  3. Transfer the Mahalito program to your PC-98 as well
  4. Insert a floppy into your drive
  5. Format the floppy if you haven’t using:
    1. FORMAT b: /M
  6. Write the image to the floppy using:
    1. mahalito e FLOPPY_IMG b:
      1. You will have two files so FLOPPY_IMG should be the name without extension. Ex: if you have DISK1.2HD and DISK1.DAT use DISK1 for FLOPPY_IMG.

Software Shortcuts


  • Change path:
    • Press ‘L’


  • Save file:
    • Press ‘F1’ then ‘S’
  • Quit:
    • Press ‘F1’ then ‘Q’
  • Save and Exit:
    • Press ‘F1’ then ‘E’