LmaOS

LmaOS is a 65C02 system kernal designed primarily for the n8 Bit Microcomputer, a homebrew computer. If you are looking for the hardware this is designed to run on, check out the n8 Bit Microcomputer.

Getting Started

To build LmaOS, you will need make and the CC65 suite or my unreleased 65C02 IDE for macOS, System On Chip (how did you get this? It's unreleased!)

From the root directory, run make:

$ make
/usr/local/bin/cl65 -t none -C LmaOS.65c02/memorymap.cfg --asm-include-dir LmaOS.65c02/include -l lmaos.rom.listing -vm --mapfile lmaos.rom.map --cpu 65C02 -o ./lmaos.rom  LmaOS.65c02/src/main.asm

To install LmaOS on an EEPROM for use in your n8 Bit Microcomputer, you will need minipro and a TL866II Plus programmer. Again, from the root directory, run make install:

$ make install
minipro -p AT28C256 -w lmaos.rom
Found TL866II+ 04.2.122 (0x27a)
Erasing... 0.02Sec OK
Protect off...OK
Writing Code...  6.78Sec  OK
Reading Code...  0.48Sec  OK

Conventions

LmaOS has a few conventions to consider. First, LmaOS uses 16 bit pseudo-registers for passing data between system subroutines: r0 through r7 (see zeropage.inc.) These registers should be considered volatile and if you use them in your program and call system functions, they could be mutated.

Second, register values at this time should also be considered volatile (with the exception of interrupt handling) so if you care about these values, push them onto the stack or store them elsewhere before calling system routines. Exactly what is used should be in the documentation for each subroutine.

Lastly, some programming conventions are:

• constants and volatile storage (.bss) are in an .inc file
• code and non-volatile storage (RODATA) are in an .asm file
• Border-line excessive use of local labels in subroutines (prefixed with @). Not all of these local labels are actually used as labels (in the assembler sense) but instead are intended as self-documenting organizational tools.

For example, many routines will have an immediate local label named @Preamble on entry to a subroutine and almost all have a @Done before RTS even when no branches jump to it. It's not strictly necessary, you could jump to the top-level label if necessary, but I like to organize the code this way.

RandomSubroutine:
@Preamble:
    PHA
    PHX
@GenerateRandomNumber:
    ...
@Done:
    PLX
    PLA
    RTS

Hardware Support

LmaOS was written primarily for the n8 Bit Microcomputer but likely needs customization, which could make it suitable for other designs as well.

System Globals

System globals that are used throughout LmaOS are declared in system.inc. This includes the system clock speed constant (needed for precise timing) and declarations for the n8 Bus ports` addresses. These follow the default memory mapping of the n8 Bit Microcomputer but you can customize them here if you changed that.

Normally, there should be no need to customize the system uptime interrupt interval unless you need higher accuracy, the routine is taking too much time for your uses, or you want to use it instead as more of a VSYNC timer for consistent frames.

Zeropage

LmaOS has carved out some of zeropage for particular uses. This includes the pseudo-registers (which are also for your program use), system uptime, the global interrupt vector, and more. See zeropage.inc for details on what is used and what is available for your own program use.

Expansion Ports

When you add, remove, or change the order of your installed expansion cards (or your system doesn't have any), you will have to recompile LmaOS.

First you will have to update the base address for system utilities to work. For example, if you've installed a VIA, in via.inc you'll want VIA_BASE to point to the correct base memory address. If you're using a n8 Bit Micrcomputer, you can just use the global n8 Bus port constant:

VIA_BASE := N8BUS_PORT2

There is system support for an ACIA, VIA, and 1602 style LCDs. If you are using different hardware for these, you may be able to just adapt what is provided for your purposes.

System utilities

LmaOS provides some system support for the ACIA, VIA, and 1602 style LCDS as well as some built-in strings utilities.

VIA

The VIA provides system timer support from Timer 1. If you want to take advantage of system uptime and jiffies, do not use Timer 1 and do not disable interrupts. By default, both I/O ports are configured for output. See via.inc and via.asm for more details.

ACIA

Basic ACIA functions are provided: ACIAGetByte, ACIASendByte, and ACIASendString. All of these routines are synchronous, meaning they could lock up the system if there is no response from a connected system. A fully working interrupt driven ACIA implementation was written if you want to comb the commit history but it was way too complicated for realworld use. See acia.inc and acia.asm for more details.

The XModem receive subroutine used by Monitaur's tx command is provided for program use as well: XModemReceive. See xmodem.inc and xmodem.asm for more details.

Strings

Some C lib like string utilities are provided including: StringLength, StringCompareN, StringCompare, StringCopy, HexStringToWord, ByteToHexString, and NibbleToHexString. See strings.asm for more details.

Pseudoinstructions

Pseudoinstructions are provided via macros. These can simplify common but rote tasks and make your code more readable but be careful when you use them. They may not always do what you think they do! You can discover them in pseudoinstructions.inc.

Monitaur

At bootup, LmaOS boots into its monitor, called Monitaur. Monitaur provides 4 basic commands: rd, wr, tx, and ex. All numeric values in Monitaur are in hexadecimal (written as $<number> throughout this document). Where noted, a word is 16 bits and a byte is 8 bits.

In general, Monitaur parses all values as a word but a command may ignore the upper nibble if it expects a byte. See monitaur.inc and monitaur.asm for more details.

Reading memory

The rd command allows you to read system memory. You provide an address (word) and, optionally, a length (byte).

>rd 0400
A9

By default, if you don't specify a length, it defaults to $1. To read blocks of memory, you can provide a length.

>rd 1000 10
AB 11 FE 6F D9 DB 2D FD 26 F4 82 FE F1 A6 95 8B

Monitaur will print $10 values per line. Length is provided as a byte and so the maximum value you can read is $100. To read the full $100 values, because of how the monitor is written, you must provide the length as $0 not $FF.

>rd 1000 0
AB 11 FE 6F D9 DB 2D FD 26 F4 82 FE F1 A6 95 8B 
25 33 49 6F 8D C0 03 3C EA D7 9F F6 0F CB ED E1 
D3 73 F8 4C C7 BA 7F B7 EB 53 5B 5E DD BC FA C1 
63 B7 E8 8F 29 91 A2 5C 13 1B E7 F7 73 72 73 BE 
B1 91 14 FC DA 5A 9F E7 D7 8F CF 7F 82 6E DB F7 
3F E7 AA 9B 1E A8 61 D9 4B 5D FF 9C 58 B7 74 BA 
7C 3D 26 FE 73 77 9F 1E 9B 5F 70 BE 75 AB 37 A3 
45 E4 B5 28 E0 E3 3D C4 D5 F1 9F 60 8F 9C 7E A3 
EE 9E F9 3E EA DF 27 EF F3 55 C8 E8 6D 1A F7 BE 
DA B3 26 C6 FE EE FD 6F 67 58 A7 79 F5 58 B5 4D 
B5 36 FA A8 66 9D B5 91 FF FA 8B B7 FF BF D3 39 
4F E7 F3 BF 39 BF D7 D0 F5 7F FF 9B E8 6C 93 9C 
2C 5C DE DD DD B3 75 7F 3E DE 7B C8 4F 4F 65 EA 
7F E4 5F 51 BF 70 B1 1B FF FF E6 D2 EA F9 77 BD 
93 EE EE 37 51 0F 8B 9E 8F E4 F9 B7 AF CE B7 BB 
FF 13 BE F9 EF FA FF EE ED 27 67 E0 5F BD 11 83

Writing memory

The wr command allows you to write to system memory (provided it is writeable!) You provide an address (word) and a value (byte) to be written.

>wr 1000 A5

Values can only be written 1 at a time through this command. There is no feedback if a write fails because you tried to write to an address that is read-only (like ROM). If you need to know if your wr succeeded, send a rd command with the address and verify the value.

Transferring data

The tx command allows you to transfer data from the connected system to the n8 Bit Microcomputer. You provide an address (word) to start writing the data.

>tx 0400
Waiting for XModem transfer...

When prompted, your connected system can send a file via the XModem Checksum (not CRC) protocol. When the transfer is completed or canceled, you will be returned to Monitaur. The tx command will write all data sent, including EOF padding ($1A from the serial app I use), it makes no attempt to discover the true EOF.

Executing programs

The ex command executes code starting at the provided address (word).

>ex 0400

ex does this by jumping to the provided address as a subroutine (JSR <address>). Your program is expected to know where it will be written (because you likely used tx and provided that address!) When your program is done executing, calling RTS will return you to Monitaur.

Note: You should always start writing your program after address $0400, as there are two $80 length buffers needed by Monitaur (for command storage) and tx (for packet storage) as well as some other bookkeeping when transferring data.

Writing Programs

The point of the n8 Bit Microcomputer and LmaOS is to write programs and see them come to life! Take a look at the following guidelines and the included example programs.

The Basics

To write programs for the n8 Bit Microcomputer (and LmaOS), copy any of the relevant include (.inc) files for your use. Set an origin for your program (I like $0400) as there is no built-in relocation scheme yet. If your program just performs a small task, remember to call RTS to return to Monitaur. If you never need to return to Monitaur, don't worry about it!

Note: When you transfer your program, the transfer location must match the origin!

Here's a very simple program that will write an alternating pattern to the VIA's 2 I/O ports:

.org $0400

Main:
    LDA #$AA
    STA VIA_BASE+PORT_A
    STA VIA_BASE+PORT_B
    RTS

.include "via.inc"

Then transfer the program using Monitaur and execute it:

LmaOS v1.0
Unauthorized access of this N8 Bit Microcomputer will result in prosecution!
>tx 0400
Waiting for XModem transfer...
Transmission successful.
>ex 0400
>

If you need to execute system provided (ROM) subroutines, copy lmaos.inc into your project. This file is not actually used by LmaOS; instead it is just a listing of the addresses of externally available system functionality.

lmaos.inc is generated each time you run make on LmaOS. This means you will have to copy this file to your program's project and reassemble each time you update your system's ROM. You can also directly include this file (and all the other system utitilies) as the example programs do in this repo. See the Makefile of the examples for how to do this.

.org $0400

Main:
@ClearScreen:
    LDA #(LCD_INSTR_CLEAR)
    JSR LCDSendInstruction
@PrintMsg:
    LDA #<WelcomeMsg
    LDX #>WelcomeMsg
    JSR LCDPrintString
@Done:
    RTS

WelcomeMsg: .asciiz "Welcome to LmaOS!"

.include "lmaos.inc"

Injecting Interrupt Handlers

If you add new hardware support or want to piggyback on the system timer interrupt, LmaOS provides a way to inject your own interrupt handlers.

First, you will need to overwrite InterruptVector (we will write CustomInterrupt in the next step):

SystemInterrupt: .res 2                         ; we'll store the original vector here

InjectInterrupt:
    SEI                                         ; turn off interrupts. we don't want them firing when we overwrite this
    COPY16 InterruptVector, SystemInterrupt     ; store original vector
    COPYADDR CustomInterrupt, InterruptVector   ; inject our interrupt
    CLI                                         ; turn interrupts back on

A couple notes about injecting your subroutine:

• as noted in the code, make sure you turn off interrupts before overwriting this value, otherwise you could jump to a random part of memory if an interrupt occurs in the middle of this operation
• be sure to store the value of the current system interrupt handler. You never know who is actually invoking your program and they may have set up their own custom handler.
• when you're done, turn interrupts back on.

Next, write your custom interrupt handler:

CustomInterruptHandler:
    BIT N8BUS_PORT_1                   ; test our custom hardware to see if this interrupt is intended for us :)
    BPL @Unhandled                     ; in this scenario highest-order bit set means we should handle it
@HandleInterrupt:
    PHA
    PHX
    ;; now do our work as quickly as possible!
    PLX
    PLA
    RTI                                ; return directly. if other interrupts are present they'll get handled :)
@Unhandled:
    JMP (SystemInterrupt)              ; let the system handle it

A couple notes about this handler:

• if you have fully handled the interrupt, call RTI. Even if there is a system timer event at the same time, it will re-raise the interrupt.
• if you didn't handle the interrupt (because your hardware didn't raise it, for example), JMP to InterruptHandleSystemTimer, the declared system interrupt handler. That will call RTI for you when it's finished.
• Be a good system citizen! Be as fast as you can and preserve any register values before either calling RTI or jumping to the system interrupt handler.

When your program is done and you want to return to Monitaur, you need to set the interrupt back to its original value. Like before, make sure you're only editing the system's interrupt handler when interrupts are off.

RestoreInterrupt:
    SEI
    COPY16 SystemInterrupt, InterruptVector
    CLI

Future Support

Planned improvements for LmaOS include:

• FAT filesystem support
• bootstrapping Monitaur and other system utilities from FAT storage
• OLED display routines
• standalone input (keyboard, controller, etc.)
• DUART support