Kernel functions

The system exposes a set of core kernel functions at fixed memory addresses located just below the 6502 vector table. This area is referred to as the jump table.

Each jump table entry is a 3-byte JMP instruction that redirects to the actual implementation of a function. This design allows system programs and user applications to call kernel routines via stable, absolute addresses, regardless of where the underlying code is actually located.

How It Works

The jump table starts at address $FFE5, with each function occupying 3 bytes (the size of a JMP absolute instruction). For example:

jsr $FFE5  ; call putstr

This instruction jumps to the putstr routine, allowing the user to print a string without needing to know where putstr is implemented in memory. This makes the jump table a forward-compatible interface to the kernel.

Overview jump table

Below is an overview of currently available jump table entries:

Label	Address	Description	Input Registers	Garbled Registers
putstr	$FFE5	Prints a null-terminated string.	X:A = High:Low pointer to string	A, X, Y
putstrnl	$FFE8	Prints a null-terminated string followed by a CRLF.	X:A = High:Low pointer to string	A, X, Y
putch	$FFEB	Outputs a single character to ACIA.	A = Character	None
newline	$FFEE	Prints carriage return and line feed.	-	None
puthex	$FFF1	Prints a byte in hexadecimal format.	A = Byte to print	None
putdec	$FFF4	Prints a byte in decimal format.	A = Byte to print	X, Y
getch	$FFF7	Retrieves a character from input buffer.	—	A

Note

The jump table is exactly the same for both the TINY and MINI boards.

Warning

Garbled refers to CPU registers whose contents are modified or overwritten by a routine and are not restored before returning. This means the calling code should not rely on the original values of these registers after the routine executes.

For example, if a function garbles register X, then any value that was in X before the function call may be lost and should not be assumed valid afterward. To preserve important register values across such calls, the caller is responsible for saving and restoring them (e.g., using PHA, PHX, PHY and their corresponding pull instructions).