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IBM PC Emulator

An IBM PC Emulator written in C using SDL3

Emulated Hardware

  • i8086 CPU
  • i8237 DMA
  • i8253 PIT
  • i8255 PPI
  • i8259 PIC
  • i8272 FDC (NEC uPD765)
  • IBM Fixed Disk Drive (XEBEC)

Emulated ISA Cards

  • MDA ISA Card (IBM Monochrome Display Adapter)
  • CGA ISA Card (IBM Color Graphics Adapter)
  • FDC ISA Card
  • XEBEC ISA Card

Building

The project is built in Visual Studio 2022 or 2026

Dependencies Github
I8086 https://github.com/tommojphillips/i8086
TOMI https://github.com/tommojphillips/TOMI
SDL3 3.2.24 https://github.com/libsdl-org/SDL
SDL3_ttf-3.2.2 https://github.com/libsdl-org/SDL_ttf
ImGui 1.92.3 https://github.com/ocornut/imgui/tree/v1.92.3
  1. Clone the repo and submodules
git clone --recurse-submodules https://github.com/tommojphillips/ibm_pc.git
  1. cd dir to ibm_pc
cd ibm_pc
  1. Fetch SDL3/SDL3_TTF VC Release
scripts\fetch_sdl3.sh

  1. Open lib\UI\vc\UI.sln, build

  2. Open vc\ibm_pc.sln, build, run.

Make sure these files are next to ibm_pc.exe or on your PATH:

  • SDL3.dll
  • SDL3_ttf.dll
  • UI.dll

If building from source, and launching through visual studio, all .dll files are already on your PATH. You only need to copy the .FON files to the same directory as ibm_pc.exe

FONT Files

Font files are required for full functionality. FONTS can be found here: https://int10h.org/oldschool-pc-fonts/download/

Description File
MDA Font 9x14 Bm437_IBM_MDA.FON
CGA Font 8x8 Bm437_IBM_CGA.FON

They have been included in the fonts\ directory of the repo.

ROM Files

ROMS are required for full functionality. ROMS can be found on 86Box's github: https://github.com/86Box/roms/

IBM PC 5150 (PC) ROMS

  • The earlier 1981 BIOSes only read the first 4 bits of SW2 resulting in a max of 544KB of RAM
  • The later 1982 BIOSes read the first 5 bits of SW2 resulting in a offical max of 640KB of RAM
Description Date Size Address Location
IBM 5150 U33 24 APR 1981 8K 0xFE000 machines\ibmpc\BIOS_IBM5150_24APR81_5700051_U33.BIN
IBM 5150 U33 19 Oct 1981 8K 0xFE000 machines\ibmpc\BIOS_IBM5150_19OCT81_5700671_U33.BIN
IBM 5150 U33 16 AUG 1982 8K 0xFE000 machines\ibmpc82\BIOS_5150_16AUG82_5000024_U33.BIN
IBM 5150 U33 27 Oct 1982 8K 0xFE000 machines\ibmpc82\BIOS_5150_27OCT82_1501476_U33.BIN

IBM PC 5160 (XT) ROMS

Description Date Size Address Location
IBM 5160 U18 16 AUG 1982 32K 0xF8000 machines\ibmxt\BIOS_5160_16AUG82_U18_5000026.BIN
IBM 5160 U19 16 AUG 1982 8K 0xF6000 machines\ibmxt\BIOS_5160_16AUG82_U19_5000027.BIN
Description Date Size Address Location
IBM 5160 U18 08 NOV 1982 32K 0xF8000 machines\ibmxt\BIOS_5160_08NOV82_U18_1501512.BIN
IBM 5160 U19 08 NOV 1982 8K 0xF6000 machines\ibmxt\BIOS_5160_08NOV82_U19_5000027.BIN
Description Date Size Address Location
IBM 5160 U18 10 JAN 1986 32K 0xF8000 machines\ibmxt86\BIOS_5160_10JAN86_U18_62X0851_27256_F800.BIN
IBM 5160 U19 10 JAN 1986 32K 0xF0000 machines\ibmxt86\BIOS_5160_10JAN86_U19_62X0854_27256_F000.BIN
Description Date Size Address Location
IBM 5160 U18 09 MAY 1986 32K 0xF8000 machines\ibmxt86\BIOS_5160_09MAY86_U18_59X7268_62X0890_27256_F800.BIN
IBM 5160 U19 09 MAY 1986 32K 0xF0000 machines\ibmxt86\BIOS_5160_09MAY86_U19_62X0819_68X4370_27256_F000.BIN

BASIC ROMS

Basic version C1.0 ROM

Basic is optional and only for the 5150 (PC). Basic comes included in the 5160 (XT) BIOSes.

Description Size Address Location
BASIC C1.0 U29 8K 0xF6000 machines\ibmpc\IBM 5150 - Cassette BASIC version C1.00 - U29 - 5700019.bin
BASIC C1.0 U30 8K 0xF8000 machines\ibmpc\IBM 5150 - Cassette BASIC version C1.00 - U30 - 5700027.bin
BASIC C1.0 U31 8K 0xFA000 machines\ibmpc\IBM 5150 - Cassette BASIC version C1.00 - U31 - 5700035.bin
BASIC C1.0 U32 8K 0xFC000 machines\ibmpc\IBM 5150 - Cassette BASIC version C1.00 - U32 - 5700043.bin
BASIC C1.0 U29,U30,U31,U32 32K 0xF6000 machines\ibmpc\ibm-basic-1.00.rom

Basic version C1.1 ROM

Description Size Address Location
BASIC C1.1 U29 8K 0xF6000 machines\ibmpc82\IBM 5150 - Cassette BASIC version C1.10 - U29 - 5000019.bin
BASIC C1.1 U30 8K 0xF8000 machines\ibmpc82\IBM 5150 - Cassette BASIC version C1.10 - U30 - 5000021.bin
BASIC C1.1 U31 8K 0xFA000 machines\ibmpc82\IBM 5150 - Cassette BASIC version C1.10 - U31 - 5000022.bin
BASIC C1.1 U32 8K 0xFC000 machines\ibmpc82\IBM 5150 - Cassette BASIC version C1.10 - U32 - 5000023.bin
BASIC C1.1 U29,U30,U31,U32 32K 0xF6000 machines\ibmpc82\ibm-basic-1.10.rom

XEBEC ROM

The Xebec ROM is required to use the Hard disks.

Description Size Address Location
Xebec 4K 0xC8000 hdd\st506\ibm_xebec_62x0822_1985.bin

Launching

Make sure these files are next to ibm_pc.exe:

  • Bm437_IBM_CGA.FON
  • Bm437_IBM_MDA.FON

Command-Line:

Switch Switch (ALT) Description Values
-config <config_file> -c <config_file> Set the config file file path
-o <offset> N/A Set the memory offset for the next ROM file. 0x00000 - 0xFFFFF
<rom_file> N/A Path to a ROM file. It is loaded at the current offset. file path
<A-D>: <disk_path> N/A Load a disk image into a drive. A, B, C, D ; file path
-disk-write-protect -dwp Write protect the next loaded disk. N/A
-disks <0-4> -ds <0-4> Number of floppy drives present. 0-4. 0 - 4
-video <video_adapter> -v <video_adapter> Selects which display adapter to emulate MDA, CGA
-ram <ram> -r <ram> Amount of conventional RAM. 16KB - 736KB
-sw1 <sw1> N/A Value of motherboard DIP switch SW1 0 - 255
-sw2 <sw2> N/A Value of motherboard DIP switch SW2 0 - 255
-model <model> N/A Motherboard model 5150_16_64, 5150_64_256, 5160
-dbg N/A Enables the debug UI N/A

Notes

  • Numbers can be written in decimal, hexadecimal (0x), or binary (0b) form.
  • RAM can be provided in KB or BYTES.
  • When a ROM is loaded, -o is automatically incremented by the files size; You can chain ROM files with a single -o <offset>
  • If a DIP switch (sw1, sw2) isnt provided; it is set automatically based on the provided config

Example:

ibm_pc.exe -o 0xF6000 cb10_1.bin cb10_2.bin cb10_3.bin cb10_4.bin bios.bin -video CGA -ram 640 -model 5150_64_256 A: msdos4_startup.img

Means:
 Load cb10_1.bin (2K file) at 0xF6000
 Load cb10_2.bin (2K file) at 0xF8000
 Load cb10_3.bin (2K file) at 0xFA000
 Load cb10_4.bin (2K file) at 0xFC000
 Load bios.bin   (8K file) at 0xFE000
 Video adapter is a CGA Card
 640KB conventional RAM installed
 Model 64KB-256KB Motherboard
 msdos4_startup.img disk is in drive A:

Configuration File:

The emulator supports configuration files written in a custom structured INI-like format called TOMI (Tom’s Object Markup INI).
It extends traditional INI syntax with support for inline structures and nested data, making it easy to define complex machine setups.

Format:

Basic Key-Value Pairs

Each setting is defined as a key-value pair.
The assignment character = associates a key with a value. Whitespace is optional.

  key1=value
  key2 = value
  key3 = 'value'
  key4 = "value"
  ...

Multiple pairs can also appear on the same line, separated by commas:

  key1=value, key2 = value, key3 = 'value', key4 = "value", ...

Structs

A struct is a collection of key-value pairs grouped together using square brackets [ ]. This allows related settings to be logically grouped.

  struct = [
    key1=value,
    key2 = value,
    key3 = 'value',
    key4 = "value",
    ...
  ]

Structs can also be written in a single line:

  struct = [ key1=value, key2 = value, key3 = 'value', key4 = "value", ... ]

Settings:

Key Type Description Values
model ENUM Motherboard model 5150_16_64, 5150_64_256, 5160
video_adapter ENUM Selects which display adapter to emulate MDA, CGA, CGA40, CGA80
conventional_ram INT Amount of conventional RAM installed 16K - 736K
num_floppies INT Number of floppy drives present 0 - 4
sw1 INT Value of motherboard DIP switch SW1 0 - 255
sw2 INT Value of motherboard DIP switch SW2 0 - 255
sw1_override BOOL Forces manual SW1 value instead of auto-calculated true, false
sw2_override BOOL Forces manual SW2 value instead of auto-calculated true, false
disk STRUCT Defines one or more floppy disk drives See below
hdd STRUCT Defines one or more Hard disk images See below
rom STRUCT Defines one or more ROM images See below
texture_scale_mode ENUM Texture sampling mode Nearest, Linear
display_scale_mode ENUM Display scaling behavior Fit, Stretched
display_view_mode ENUM Display framing mode Cropped, Full
correct_aspect_ratio BOOL Enables 4:3 aspect correction true, false
emulate_max_scanline BOOL true, false
allow_display_disable BOOL Allow software to disable the display true, false
delay_display_disable BOOL Delay the display disable (prevents flickering due to software disabling and enabling the display rapidly) true, false
delay_display_disable_time INT The display disable delay time milliseconds
mda_font STRING Path to the MDA font file file path
cga_font STRING Path to the CGA font file file path
dbg_ui BOOL Enables the debug UI true, false

ROM settings:

Key Type Description Values
path STRING Path to the ROM file file path
offset INT The offset to load the ROM file 0x00000 - 0xFFFFF

DISK settings:

Key Type Description Values
path STRING Path to the Disk file file path
drive INT The drive to load the disk file in. A, B
write_protect BOOL write protects the drive true, false

HDD settings:

Key Type Description Values
path STRING Path to the Disk file file path
drive INT The drive to load the disk file in. C, D
geometry STRUCT Geometry override for RAW images See below
type ENUM Type override for RAW images Type1, Type2, Type13, Type16
  • If loading a raw hard disk image; some geometries have ambiguous file sizes and cant determine the geometry from the file size alone.
  • Use geometry OR type parameter of the hard disk to tell the emulator what the disk geometry is.
  • If both geometry and type are supplied then the geometry is used to determine it.

GEOMETRY settings:

Key Type Description Values
c INT Max cylinders of the hard disk
h INT Max heads of the hard disk
s INT Max sectors of the hard disk

Type settings

See Hard disk drive geometries

Notes

  • Each disk, hdd or rom struct can appear multiple times to define multiple drives or ROMS.
  • Numbers can be written in decimal, hexadecimal (0x), or binary (0b) form.
  • Comments start with ; and extend to the end of the line.
  • Missing values fall back to defaults defined in the emulator.
  • If a sw1_override or sw2_override is false; the associated SW is set automatically based on the provided config
  • RAM must be provided in BYTES.

Example INI:

; ----------------- PC Config ------------------
model = '5150_64_256'         ; 5150_16_64, 5150_64_256, 5160
video_adapter = 'CGA'         ; MDA, CGA, CGA40, CGA80
conventional_ram = 0x40000    ; 0x4000 - 0xB8000 (16K - 736K)
num_floppies = 2              ; 0, 1, 2
; ----------------------------------------------

; --------------- DIP SWITCHES -----------------
sw1_override = false
sw2_override = false
sw1 = 0b00000000
sw2 = 0b00000000
; ----------------------------------------------

; --------------- Floppy Disk Drives ----------------
disk = [ drive = 'A', path = 'floppies/msdos4_startup.img'  ]
disk = [ drive = 'B', path = 'floppies/msdos4_working6.img' ]
; ----------------------------------------------

; --------------- Hard Disk Drives ----------------
hdd = [ drive = 'C', path = 'hdds/hdd.vhd'  ]
hdd = [ drive = 'D', path = 'hdds/raw_hdd.raw', type = 'Type13'  ]
; ----------------------------------------------

; -------------------- ROMS --------------------
rom = [ address = 0xF6000, path = 'roms/5150cb10_1.bin' ]
rom = [ address = 0xF8000, path = 'roms/5150cb10_2.bin' ]
rom = [ address = 0xFA000, path = 'roms/5150cb10_3.bin' ]
rom = [ address = 0xFC000, path = 'roms/5150cb10_4.bin' ]
rom = [ address = 0xFE000, path = 'roms/BIOS_IBM5150_27OCT82.BIN' ]
rom = [ address = 0xCC000, path = 'roms/expansion_rom.bin' ]
; ----------------------------------------------

; --------------- Display Config ---------------
texture_scale_mode = 'Nearest' ; Nearest, Linear
display_scale_mode = 'Fit'     ; Fit, Stretched
display_view_mode = 'Cropped'  ; Cropped, Full
correct_aspect_ratio = true
emulate_max_scanline = true
allow_display_disable = true
delay_display_disable = true
delay_display_disable_time = 200
; ----------------------------------------------

; --------------- Display Fonts ----------------
mda_font = 'fonts/Bm437_IBM_MDA.FON'
cga_font = 'fonts/Bm437_IBM_CGA.FON'
; ----------------------------------------------

; ------------------- DEBUG --------------------
dbg_ui = false
; ----------------------------------------------

XEBEC

2 Image formats are currently support for hard disks. VHD or RAW/IMG images.

VHD

If using a VHD the geometry of the disk will be automatically detected since the VHD Stores the disk geometry. Using the VHD format allows you to browse a FAT12/FAT16 formated Hard disk in your Host OS like any other VHD.

  • A VHD cannot be saved while it is mounted in the Host OS.

  • A VHD can be loaded while it is mounted in the Host OS.

  • DOS will not see changes to the VHD made by the Host OS until a reboot of the emulator has been done. DOS caches the FAT and accessed files in RAM. Any new files created or modified might not show up until a reboot of the emulator. Using a program i have created i have been able to get around this issue. See FDFLUSH.COM

RAW IMAGE

If using a RAW image, Some hard disk geometries cant be determined from the file size alone due to multiple hard disk geometries having the same total size.

Hard Disk Geometries

Type Geometry Total size (MB)
Type1 C = 306, H = 4, S = 17 10.16MB
Type2 C = 615, H = 4, S = 17 20.41MB
Type13 C = 306, H = 8, S = 17 20.32MB (ambiguous)
Type16 C = 612, H = 4, S = 17 20.32MB (ambiguous)

Creating your Hard disk

[TODO]

Formating your Hard disk

Using FDISK/FORMAT

  • Load the unformatted hard disk in the emulator. For this demo; the hard disk is in Drive C:
  • Boot a DOS floppy in drive A: that has the FDISK and FORMAT utilities
  • Run FDISK to create a partition table on the hard disk.
  • Run FORMAT C: /s to format the hard disk and copy the system files to it.
  • Eject the floppy from drive A:
  • Reboot the emulator.

FDFLUSH.COM

This program invalidates the FAT and forces all floppies and hdds to be re-cached next time they are accessed. This allows changes made by the host OS to show up without a reboot of the emulator. This program can be found in the programs\ directory of the repo.

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An IBM PC Emulator

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