JiffyDOS Kernal Upgrade

**JiffyDOS Kernal Upgrade**
Caption	JiffyDOS ROMs fitted to a C64C motherboard and 1541-II drive mechanism
Type	Fast-serial DOS & KERNAL replacement (ROM upgrade)
Designer	Mark Fellows (original author); Creative Micro Designs (CMD); Retro Innovations (current licence-holder);
Manufacturer	CMD (1987 – 2001) • Retro Innovations (2004 – present under licence)
First released	1985
Latest revision	Still in production
Operating voltage	—
Layers / PCB	—
Compatibility	C64, C64 C, SX-64, C128, C128 D, VIC-20 CR · 1541/1541-II/1541 C · 1571 · 1581 · many third-party IEEE/serial drives
Features	6-10× faster IEC bus, full DOS Wedge, extended command set, function-key macros, bug-fixed drive ROMs, on/off toggle switch
Model No.	JiffyDOS 6.01 (1990) – current

JiffyDOS is a drop-in firmware upgrade for Commodore 8-bit computers and their disk drives. By replacing the system’s KERNAL ROM and the DOS ROM inside each attached drive, JiffyDOS rewrites the serial-bus protocol to eliminate the hand-shaking delays that plague stock IEC transfers. The result is a 6- to 10-fold speed-up on a 1541 and proportional gains on 1571/1581 drives.

Because the code lives in ROM it consumes no RAM, needs no cartridge slot, and retains virtually 100 % software compatibility. A small rocker or slider switch is normally fitted so the user can revert to the original ROM image if a tape-based program, copy-protection routine, or hardware fast-loader demands it.

Key Features

Fast serial protocol – burst-style block transfers with sector interleave control on 1541/71/81; typical 202-block load falls from 124 s → 12 s on a 1541 :contentReference[oaicite:1]{index=1}
Complete DOS Wedge built in – shorthand commands (`@`,`$`,`%`,`↑`,`←` etc.) for directory listing, file copy, scratch, validate, drive number change
Extra JiffyDOS-only commands – head-rattle suppression (`@B`), screen-to-printer dump, BASIC *UNNEW* (`@O`), disk copy, gap-size tuning
Function-key macros pre-loaded with useful disk operations
Bug-fixed Commodore ROMs – 1571 relative-file and autoboot fixes, 1541 buffer-overflow patch, 1581 improved *VALIDATE*
Hardware toggle – completely disables JiffyDOS for maximum compatibility, unlike cartridge fast-loaders that can only be soft-disabled
No extra cabling – uses the standard six-wire IEC serial bus; works in parallel with cartridges, REUs, user-port devices, RAMLink, etc.

Performance

**Typical 1541 / 1571 / 1581 transfer times (JiffyDOS 6.01)**
Operation	⏱ Stock 1541	⏱ JiffyDOS 1541	Gain
Load 202-block PRG	124 s	12 s	× 10.3
Save 100-block PRG	75 s	24 s	× 3.1
Read 125-block SEQ	84 s	15 s	× 5.6
Write 100-block SEQ	81 s	27 s	× 3.0

On a 1581 the absolute gains are slightly smaller (fewer mechanical delays) but the proportional speed-up remains 5-8×.

Supported Machines

Computers

Commodore 64 / 64C
Commodore SX-64 (portable)
Commodore 128 / 128D (speeds both C64-mode and native C128-mode)
Cost-reduced VIC-20 (requires two-ROM set)

Disk drives

1541, 1541-C, 1541-II, 1571, 1581 and more than twenty third-party compatibles (Excelerator+, OC-118, MSD 1/2, Indus GT, etc.)

Physical Package

Computer ROM – usually a 27128 (16 KB) EPROM on a 28-pin adapter PCB; early C64s with 24-pin 2364 mask ROMs need a pin-header adapter or board trace-cut.
Drive ROM – 27128/27256 EPROM; 1541 C and 1541-II need solderless swap, older 1541 drives may need a jumper moved from “24-pin” to “28-pin”.
Toggle switch – DPDT or SPDT depending on board, mounted through RF shield or case rear, selects between stock and Jiffy images.

Retro Innovations sells both pre-programmed, legally licensed ROMs and download images for owners with their own EPROM burner. A computer set costs ~US $27 and each drive ROM US $19 (2024 price list).

Installation

Disassemble the computer/drive; locate the KERNAL or DOS ROM (consult the service manual for your board revision).
Note the notch orientation, carefully lever the old IC from its socket.
Fit the EPROM adapter—ensure pins are fully seated and the notch faces the same direction.
Mount the toggle switch and wire it to the adapter (or fit the supplied PCB-mounted switch).
Re-assemble and power-on. Type @ (return) – a drive error-channel read – to confirm the JiffyDOS banner.

A full illustrated guide (PDF) accompanies each ROM set and is mirrored on C64-Wiki .

Drive-only Upgrade?

You may install JiffyDOS in drives only; they remain perfectly usable from stock C64s (they fall back to Commodore serial timing when the computer side is stock). However, the computer’s KERNAL must be upgraded to enjoy the speed increase.

Command Reference

JiffyDOS merges Commodore’s DOS Wedge and additional shorthand. Highlights include:

Syntax	Action
`@$`	Show disk directory without clobbering BASIC program
`/prog`	Load BASIC program “prog”
`↑prog`	Load and RUN BASIC program
`%prog`	Load machine-language file to its stored address
`@C:new=old`	Copy file on same disk
`@B`	Disable 1541 head-rattle on error
`@O`	UNNEW – recover BASIC program after NEW
`@UJ`	Warm-reset the drive

A ten-page quick-reference card and the full 64-page user’s manual are freely downloadable from the publisher.

Compatibility

Software: near-universal; even copy-protected titles load because JiffyDOS automatically detects and drops to slow-serial when the computer or drive requests it.
Cartridge fast-loaders: OK – the first cartridge to take over disables JiffyDOS until the next reset.
SD2IEC / Pi1541: JiffyDOS transfer protocol is fully supported, giving SD card–based solutions similar speed gains.
Tape software: JiffyDOS disables the cassette port while active; switch back to “Stock” to load tapes.

Licensing

Mark Fellows retains copyright; Retro Innovations holds the exclusive distribution licence. Purchasing a ROM or image therefore is required for legal use – the ROMs are not abandon-ware.

Related Projects

SJLOAD – software fast-loader compatible with JiffyDOS protocol
S-JiffyDOS – patched ROM offering soft-switch and RAM-resident operation
JaffyDOS – SD2IEC-optimised fork with long-filename directory lister