James Routley

os8088 1.0

A Macintosh System 1-style desktop for the Intel 8086, written entirely in real-mode assembly and booted from a floppy. Overlapping windows, pull-down menus, a serial mouse, loadable programs -- and pre-emptive multitasking, which the real 1984 Macintosh never had.

pixels: 640x480
colors: 16
of RAM: 256K
byte kernel: 78,950
task slots: 12
pre-emption: 18.2Hz

The os8088 desktop: a menu bar across the top, a Note Pad window with typed text, a Clock counting up, a Bounce window, the Control Panel showing a pre-emptive/cooperative choice, and the Task Manager with a CPU history graph -- five programs running at once, with a tile for each in the dock along the bottom. — **Five programs at once, on a CPU from 1978.** A Note Pad loaded off the software floppy, a ticking Clock, a bouncing ball, the Control Panel and the Task Manager, each a separate instance with its own dock tile. The Task Manager reports the machine at 70% busy and 147K of RAM in use.

What it is

os8088 boots from the first sector of a floppy into a graphical desktop. There is no DOS underneath it and no command line anywhere in it -- the 512-byte boot sector loads a 40KB kernel, the kernel works out which display adapter the machine has and switches it into graphics, borrows the video card's own 8x8 character set, and puts up a menu bar.

Everything after that is the interface you remember from a 1984 Macintosh: windows you drag by the title bar with a rubber-band outline, menus you pull down and drag through, a close box and a minimize box, icons for each floppy drive, and a dock along the bottom with one tile per running program. Programs load from a second floppy and run as first-class applications, several at a time.

The whole thing fits in 256K of RAM on an Intel 8086 or 8088 -- the processor in the original IBM PC and PC/XT. It is a hobby project, it is finished enough to use, and the source is 112,792 lines of NASM you can read in an afternoon.

By the numbers

Every figure below comes from the build, not from memory.

Verified specifications of os8088
Kernel size	78,950 bytes	The scheduler, window manager, VGA driver, mouse driver, sound layer, file manager, loader, Task Manager and Control Panel, all of it.
Boot sector	512 bytes	LBA-to-CHS translation and retrying BIOS floppy reads. The build fails if it assembles to anything else.
Segment footprint	53,842 bytes	Code plus every buffer the kernel reaches through DS. It must fit the 64K segment -- currently 11,694 bytes of headroom. The task stacks, the disk buffers and every loaded program live outside the segment and are not counted here.
Lines of assembly	112,792	Hand-written real-mode 8086. No C, no linker, no runtime library.
Display	VGA, Hercules or CGA	Probed at boot. VGA mode 12h is 640x480 in 16 colors -- four 1-bit planes, 80 bytes per scanline; Hercules is 720x348 and CGA 640x200, both monochrome and banked. Drawn straight to the framebuffer on all three.
Back buffer	optional, off by default	An off-screen copy of the screen is 153,600 bytes, more than half the machine's memory, so a 256K machine always draws straight to the display, as the 1984 Macintosh did. At 500K or more the Control Panel can switch one on.
Tasks	12 slots, 1,536-byte stacks	Each Clock and each Bounce window is its own pre-empted task.
Context switch rate	18.2065 Hz	The PC's stock timer rate. Every tick the handler saves nine registers, swaps the stack pointer and returns into a different program.
Loadable programs	a segment each	Packages loaded from a second floppy into their own regions of conventional memory -- about 107K of arena on a 512K machine, 233K at 640K -- several resident at once, each freeing its memory when closed.
Mouse	1200 baud, 7N1	A Microsoft serial mouse on COM1, three bytes per report, decoded in an interrupt handler that also draws the cursor.
Target hardware	IBM PC/XT, 8088 at 4.77MHz	256K of RAM, a serial mouse, and any of a VGA, Hercules or CGA card. Repaints are slow enough to watch happen.
Instruction set	8086 only, enforced	`cpu 8086` plus `-w+error`, so anything newer is a build failure. A 1978 instruction set, mechanically guaranteed.

What it does

Four of the things it is actually good at. There are more on the screenshots page.

Two Clock windows counting up and two Bounce windows with squares moving around inside them, all animating at the same time. — **Nothing has to cooperate.** Two clocks keep time and two balls keep moving while you type into something else. The timer interrupt fires 18.2 times a second, saves whichever task was running, swaps the stack pointer and returns into the next one -- about thirty instructions. The Macintosh of the period could not do this at all: System 1 ran one application at a time, and when MultiFinder arrived in 1987 it was cooperative.

A Note Pad window staying still while a one-pixel rectangular outline is dragged across the desktop, then the window jumping to the outline's position. — **The window does not move until you let go.** A one-pixel XOR outline follows the pointer and the window snaps to it on release -- because redrawing a whole window on every mouse report was out of the question at 4.77MHz, exactly as it was in 1984.

The Disk file manager showing the GAMES folder -- MINES.O88, SOLITAIR.O88 and ARKANOID.O88, each with its own icon -- with a Minesweeper window open on top showing a partly revealed 9 by 9 grid with colored numbers and red flags. — **Programs come off a second floppy.** Double-click a file in the Disk window and it loads into a segment of its own and runs as a real application. Minesweeper is 1,510 bytes including its icon. Several packages -- or several copies of one -- can be resident at the same time.

The Task Manager window showing a CPU percentage, a history graph swept across its full width, a RAM bar reading 39K of 639K, and a list of running instances with their state and CPU share. — **It watches itself.** A live CPU meter with a scrolling history graph, a RAM readout, and one row per running program. Apps without their own task get their window callbacks timed at the dispatch site and billed to them, so the rows still add up to one total.

All the screenshots

It runs on the real thing

Everything above was captured in an emulator. In August 2026 someone wrote the disk images onto actual floppies and booted them on three period machines -- a 1981 IBM 5150, a Toshiba T1100 Plus and a 286 -- and photographed the results.

A green phosphor IBM 5151 monitor showing the os8088 desktop with a Note Pad, an About box, a Clock, a Task Manager, a Minesweeper board and a Disk window open at once. — **Six programs at once on a 1981 IBM PC.** A real 5150 driving a Hercules card at 720x348, booted off a 360K floppy, with the machine's own card list typed into a Note Pad that was loaded from that same disk. Photographed by Elendilon, who also reported that it is no slower here than it is under emulation.

os8088 on real hardware

The history it is arguing with

In February 1985 Digital Research shipped GEM 1.0 for the IBM PC: a Macintosh-style desktop with overlapping windows and icons, running on stock XT hardware. Apple sued. The settlement led to GEM Desktop 2.0 in 1986, which removed overlapping windows and desktop icons on the PC -- while the Atari ST version, not covered by the agreement, kept them.

So the PC did briefly have this, and then it did not. os8088 is a rebuild of roughly that idea, from scratch, forty years later, on the same class of machine -- plus the pre-emptive multitasking that neither GEM nor the Macintosh of the period offered.

What it is not

It is not DOS and it does not run DOS programs. There is no DOS underneath it; it owns the machine from the boot sector up.
It has no memory protection. The 8086 has none to offer -- every program shares one 64K segment with the kernel, which is precisely what makes a context switch cheap enough to be worth doing.
There are no file handles and no seeking. The disk API moves whole files at a time -- write, read, delete, rename -- which is the largest subset that stays honest with no disk cache and twelve pre-empted tasks.
There is no networking.
It is a hobby project, not a product. It is also complete enough that you can sit down and use it, which is the whole point.

Try it

The browser demo boots the real floppy image in an emulated PC -- same bytes you would write to a disk. If you would rather run it yourself, the download page has 360K images for period hardware and 86Box, and 1.44MB images for QEMU.