I started by creating a new ld65 linker configuration which links code to run from address $0400 onwards, which is where programs are loaded into RAM.

I thought I was almost done at this point, but it turns out that writing a “Hello World” test program to run from RAM is easier said than done. The ASCII SUB character ( 1a in hex) fills out unused bytes at the end, since everything sent over XMODEM needs to be a multiple of 128 bytes. The screen capture shows 256 bytes of memory (2 packets in XMODEM). Everything here is running on my desk, so I’m not too worried about transmission errors.Īfter a few iterations, I was able to upload placeholder text to RAM. shell_rx_print_user_program – prints the first 256 bytes of the program for verification.shell_rx_sleep_seconds – sleeps for the number of seconds in the A register.The other routines mentioned in this source listing are: The index in the current packet is stored in the Y register, allowing each incoming byte to be written to memory with one operation, which is called in a tight loop. It uses a pointer to the program’s location, which is incremented 128 each time a packet is accepted.

This code is the first time that I’ve used the “Zero Page Indirect Indexed with Y” address mode. Lda #1 wait a moment (printing does not work otherwise.) Lda USER_PROGRAM_WRITE_PTR This next part moves write pointer along by 128 bytesīeq #$00 If low byte != 0, set to 0 and inc high byte Jsr acia_recv_char Inverse block numberīne acia_recv_char Checksum - TODO verify this and jump to shell_block_nak to repeat if not mathingīeq #$01 SOH char, next block on the way Jsr shell_rx_receive_with_timeout Check in loop w/ timeoutĬmp #$01 If we do have char, should be SOHīne Terminate transfer if we don't get Receive one block Lda SPEAKER Click each time we send a NAK or ACK USER_PROGRAM_WRITE_PTR = $00 ZP address for writing user program USER_PROGRAM_START = $0400 Address for start of user programs recives a file over XMODEM protocol, and loads it into memory This process is repeated until the end of transmission (EOT) is received. All padding around each packet is discarded, and the payload is written to memory. The process involves ASCII NAK character repeatedly until a SOH character is received, which indicates the start of a 128 byte packet. The entire receive process is around 60 lines of assembly code. Ldy ACIA_STATUS check ACIA status in inner loopīne no byte received in ACIA_RX get byte from ACIA data port Like acia_recv_char, but terminates after a short time if nothing is received I already have routines for reading and writing characters from serial. To start the transfer, I found it useful to write a method for reading a character with a time-out.

rx, to receive a file over XMODEM and load it into memory.

I have the beginnings of a shell for running commands from ROM, so I implemented two new commands: This is partly to make sure I understand it, and partly to keep the software licensing of my ROM as clear as possible. There are existing 6502 assembly implementations which I could have used, but I decided to work from the description on the Wikipedia article instead.

Implementing XMODEM was the easy part of this process. This works for me, since it’s the only interface I’ve got.įor what it’s worth, XMODEM is also era-appropriate technology for this retro computer, since it was first implemented in 1977. All I need right now is the ability to place some data (machine code) into RAM, so that I can execute it.Īn important note that is that modern file transfer protocols would normally run over TCP/IP, while XMODEM runs over a serial connection. I chose XMODEM because it’s the simplest protocol which is compatible with modern terminal software. This blog post covers changes which I made to implement the XMODEM protocol, allowing me to upload programs over a serial connection. The software on this computer is currently very limited, and re-programming an EEPROM chip to test changes is a slow process. I recently built a 6502-based computer from scratch, and I’m using it as a platform for testing 6502 assembly code.

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