JiffyDos notes (work in progress)

Last Updated or created 2023-05-23

JiffyDOS is an enhanced DOS for the C64. The software is programmed onto ROM chips that replace the Kernal ROM chip on the motherboard and the DOS ROM chip in the disk drive. JiffyDOS is intended to provide greater speed, commands and convenience than on stock systems.

The 1541 drive is a computer on its own, using a 6502 and VIA chips.
(See other pages) (C64 uses a 6510, that is the same slightly modified version of the 6502)
A cool example of the drive being an OS/computer on its own:

Some notes:

  • I want to use a larger rom and using the higher address lines as kernal selector. Address line A13 and A14 can be used as selector
  • There is a schematic out there using runstop at boottime to do de selection of the rom part

Did I misspell kernel? NO
(Below from Wikipedia)

The KERNAL was known as kernel inside of Commodore since the PET days, but in 1980 Robert Russell misspelled the word as kernal in his notebooks. When Commodore technical writers Neil Harris and Andy Finkel collected Russell’s notes and used them as the basis for the VIC-20 programmer’s manual, the misspelling followed them along and stuck.

Original Kernal: 901227-03
8-kilobyte 2364 ROM 4K * 8 bits PROM

28C265 = 32K * 8bits

Diffference in ROM size AND there are some other pin placements.

Romselect should be /(a15 * a14 * a13) depending on ram/rom switch.


$E000-$FFFF – ROM

KERNAL ROM or RAM area (8192 bytes); depends on the value of bits #0-#2 of the processor port at memory address $0001
$FFFA-$FFFF – hardware vectors

%x0x: RAM area.

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