C64-dev/samples/kernal-os-skeleton

Kernal-Resident C64 OS Skeleton

This sample shows a practical starting memory layout for a Commodore 64 OS that keeps the KERNAL ROM visible so you can continue using routines like CINT, BSOUT, GETIN, LOAD, and SAVE.

Memory Plan

Use $0001 with low bits %110:

• LORAM = 0
• HIRAM = 1
• CHAREN = 1

That gives you:

• $A000-$BFFF = RAM under BASIC ROM
• $C000-$CFFF = always-visible RAM for your OS core
• $D000-$DFFF = I/O visible
• $E000-$FFFF = KERNAL ROM visible

In practice, the commonly seen value is $36, but this sample preserves the upper cassette-control bits and only changes the low three banking bits.

Recommended Layout

• $0002, $00FB-$00FE = fast zero-page workspace
• $0200-$03FF = vectors, buffers, light system workspace
• $0400-$07E7 = screen RAM
• $0801-$9FFF = BASIC area during development, or additional RAM if you fully replace BASIC usage
• $A000-$BFFF = banked work RAM, modules, command buffers, filesystem workspace
• $C000-$CFFF = OS core and resident code
• $E000-$FFFF = KERNAL ROM, still callable through $FFxx

Why This Is a Good First Setup

It lets you:

• keep all normal KERNAL calls available
• avoid writing your own screen, keyboard, and IEC routines on day one
• gain 8 KB of extra RAM at $A000-$BFFF
• keep your OS core in safe always-RAM at $C000

What The Sample Does

• loads at $0801 as a normal BASIC-start PRG
• boots through BASIC line 10 SYS2061
• switches memory to BASIC-off / KERNAL-on / I/O-on
• writes a live work byte at $A000 and a small signature starting at $A001
• uses a file buffer at $A100 for simple shell LOAD/SAVE operations
• uses KERNAL routines for screen output and keyboard input
• lets you inspect the current banking mode and a workspace byte in $A000
• restores normal memory mapping before returning to BASIC

Build

build.bat

Run On A C64 Or In VICE

Because this program now starts in normal BASIC memory at $0801, load and run it like a standard BASIC program:

LOAD "KERNAL_OS.PRG",8
RUN

If you want to jump into it manually after loading, the BASIC stub runs:

SYS 2061

If you are in VICE:

x64sc -autostart kernal_os.prg

Then use these CLI commands:

• HELP to show the command list
• MEM to show $0001 and the live work byte at $A000
• INC to increment the work byte at $A000
• DUMP to show $A000-$A00F
• ZERO to clear the first 16 bytes of the workspace
• SIGN to rewrite the signature starting at $A001
• SCREEN to show the current C64 screen dimensions from the KERNAL
• DIR to list only the filenames on the current target
• STATUS to read and print the drive status channel on the current target
• INIT to send the standard I initialize command to the current target
• ECHO text to print text and load the shell buffer at $A100
• TOUCH filename to create an empty sequential file
• TYPE filename to print a sequential file from the current target
• LOAD filename to load a sequential file into the shell buffer at $A100
• SAVE filename to save the current shell buffer from $A100
• DEVICE n or DEV n to set the current device number, usually 8-15
• DRIVE n to set the current drive number, usually 0 or 1
• DEL filename to scratch a file on the current target
• ERASE filename as an alias for DEL
• CLS to clear the screen
• ABOUT to reprint the layout explanation
• EXIT to restore normal memory mapping and return to BASIC

Notes On The New File Commands

• TYPE, LOAD, and SAVE are implemented as sequential-file shell commands
• LOAD filename in this sample does not replace the running OS core with a machine-language PRG
• instead, it reads file data into the buffer at $A100
• SAVE filename writes the current buffer back out as a sequential file
• TOUCH filename creates an empty sequential file by opening it for write and closing it immediately
• ECHO text also fills the $A100 shell buffer so it can be saved

Simple Pipe Support

The shell now supports a simple redirection pattern for ECHO:

ECHO HELLO WORLD > SAVE TESTFILE

That:

1. copies HELLO WORLD into the shell buffer at $A100
2. prints the text
3. rewrites the rest of the command line to SAVE TESTFILE
4. runs the save command immediately

This is intentionally small and predictable rather than a full Unix-style pipe system.

That keeps the sample safe and predictable while still showing how a KERNAL-based OS shell can manage files and a current DOS target.