tinyload65

small serial bootloader for 65xx systems

• convenient software upload for fast development cycles
• remote debugging
• system startup delayed by less than 1 second
• no additional hardware required
• small code size
  • ~170 bytes (software upload only)
  • ~220 bytes (full debug support)
• can be integrated into existing firmware

inspired by urboot and avrdude by Stefan Rueger (https://github.com/stefanrueger).

disclaimer

The content of this repository may not be used for training AI or machine learning models, including but not limited to large language models (LLMs).

status

• target component fully implemented
• basic functions of the host component implemented
• little tested so far, but everything seems to work

overview

bootloader (tinyload65)

• small resident application
• invoked by reset vector
• waits for host commands on start-up
• allow the host to read and write the target memory
• normal system boot after short timeout (0.7 s)

in-system debugging

• hook nmi and/or brk vector
• give host full control of the target memory and processor state

interface

• software-only serial interface
• 57600 baud, half-duplex @ 1MHz cpu clock
• two port lines required (rx and tx)
• reset controlled by dtr
• nmi controlled by rts (optional)
• interface can be used as terminal port after start-up
• 6-pin debug connector (rx, tx, reset, nmi, gnd, vcc)

host-based uploader (tinydude65)

• small control application
• command line interface
• upload and download data
• test nmi functionality

host-based debugger (tbd)

• interactive host application
• trigger nmi
• manage breakpoints
• memory dump
• dissassembler
• single-stepping
• debug log

requirements

interface

• 1 port line for rx (bit 7 required)
• 1 port line for tx (bit 0 prefered)
• port may be re-used during normal operation

zeropage use (startup only)

• 3 bytes temporary data

ram usage (startup only)

• 256 byte ram for packet buffer
• can be reduced if required by using a smaller packet size

basic features

• identify bootloader version
• write target memory
• write reset, nmi and irq vector

optional features

• identify target system
• read target memory
• read and write reset, nmi and irq vectors
• read and write registers

protocol

general

• software serial receive requires very tight timing
• block size 256 bytes max.
• no checksums
• written data may be verified via read command if required

packet format

• packet size (1 byte)
• packet data (1 .. 255 bytes)

timeouts

• serial timeout 0.7 seconds
• timeout terminates bootloader

error handling

• unknown commands are ignored
• invalid packets terminate bootloader
• no parameter checks

basic messages

reset >

R <version_lo> <version_hi>  [ <device_data ...> ]

• signal reset signal to host system
• send protocol version
• send target device data (optional)
• target is now ready to receive commands

write <

W <addr16> <data ...> 

• write data to target memory (252 bytes max.)
• target system responds with ack

ack >

. 

• acknowledge host command

extended messages

nmi >

N <addr16>

• signal nmi signal to host system
• addr16 points to register values (sp, y, x, a, st, pc)
• host system may read and modify registers

brk >

B <addr16>

• signal brk signal to host system
• addr16 points to register values (sp, y, x, a, st, pc)
• host system may read and modify registers

read <

R <addr16> <size8>

• read memory at addr16
• target system responds with message containing requested data
• may also be used to prevent bootloader timeout

bootloader api

• tx_byte: send one byte via serial interface

wiring

pa0     out     serial      tx          use bit 0 for optimized code
pa1     
pa2     
pa3     
pa4     
pa5     
pa6     
pa7     in      serial      rx          bit 7 required for 57600 baud