Nezha : Using syscalls for IO

 It was back in July I first posted about my Nezha RISC-V SBC.  At the time I was pleased to have a solid, reliable Debian build and excited with the ability to write native RV assembly programs and run them  at the command line without the usual inconvenience of cross-compilation and loading.

I purchased Anthony Reis' (AR) beginners guide to RV assembly which provides a gentle learning curve for me.  It starts with a chapter on machine language which is interesting as it describes the format of some types of instructions ().  I am hoping not to do too much low level debugging but this info provides an insight into the RV design, in which instructions are standardised to facilitate their implementation on a variety of hardware platforms.

One of the problems with an instructional RV book is that people have a range of environments in which they write programs.  Many would use a QEMU RV VM, some would have a smaller RV machine to load executables onto and a lucky few like me have an RV linux SBCfor native compilation. As AR starts on the assembly language tutorials he provides his own program which functions as an RV-32 assembler and simulation environment.  This provides input output to the screen using his own bespoke functions.  For example two instructions sin and sout are provided to input / output a string to / from a program.  A lot of assembly programming deals with peripherals so this isn't very helpful (although it is the correct approach for a general purpose tutorial / textbook).

The first thing I need to do is to work out how to implement the same functionality in my linux / gcc environment so that I don't need to use AR's assembler / io functions.  Previously I was able to use Stephen Smith's hello world example which uses a linux system call (ecall) to output a string.  Instead of using ecall it is possible to to the call statement, so that you can use the C function name rather than a numeric syscall number.  RV registers a0, a1, a2, ... registers are used for the parameters.  I was very pleased with my first attempt, shown below, which calls printf to display a string containing an integer parameter.  I needed to link it with a linker option "-no-pie" to include the necessary library modules for terminal output.

Once we have this mechanism it is easy to provide simple macros with the same name and parameter as those used in Anthony Reis book.  Now my first program example, which adds two numbers together looks just like the one in the book.

DIY Frequency Counter

 

I am concerned with my 6502 reliability now that I have increased the clock speed to 1MHz and I want the ability to slow the clock down (again).  I could revert to my arduino Nano clock but this is quite a bit slower.  Instead I decided to use a decade counter so that I can slow the clock by a factor of 10, 100, 1000 etc.  Chips are ridiculously chep and I can easily add one next to the clock on the breadboard.
It occurred to me that I need to measure my clock speed to be sure the counter is having the expected effect.  It would also be very useful to check the nano clock generated frequenct if I am using it.

Ebay shows some cheap kits alongside expensive "professional" equipment.  I only need a rough idea so a kit is perfect.  It comprises TTH (through the hole) components, 5 x seven segment displays for output and a PIC16 microcontroller for measurement.

The only instruction provided is a link to a youtube video.  It turns out to be very helpful.  Kev, the guy who provides it, makes a board whilst talking, it takes about 5 minutes.  It is reassuring and charming that he is quite amateurish.  There are a couple of important points he makes, firstly how to solder the extra capacitor to improve low frequency measurement and secondly a reference / link to operating instructions he has found.

The board is very nice to solder with well-spaced components and generous solder pads.  I built it and it works.  It accurately tested a 20MHz oscillator as having a frequency 20.064 MHz.  Next I measured the clock speed on my W65C02SXB board, which I understood was running at 8MHz.  The measurement was 1.8MHz, looking at the clock chip on the board I noticed that this is correct - so the tester has already proved itself useful.

In conclusion, I am very pleased with this circuit, it is always nice to solder useful things.  I am now ready to face my next 6502 test.

W65C02SXB : WDC to the rescue

 

I am a little nervous about my 6502 system.  I moved beyound the "use wires to connect hardware on breadboard" stage so time ago and I am more intereseted in software development now.  In fact I am aiming to run C and BASIC programs on my system next.  Underneath the hardware is working but subject to the vagaries of bad connections or deficiencies in programming due to my incomplete understanding.  The recent increase in clock speed (or some undiscovered unknown issue) has led to the system becoming frustrating to work with and almost unuseable.

Whilst considering buying a circuit board to formalise my hardware (for example from dbuchwald) I came across a development board from WDC  which perfectly matches my requirement. It was released in 2014, apparently as an educational board and is still available. WDC have been the main player keeping 6502 hardware ecosystem alive for many years now so a board from them is likely to be good.

The W65C02SXB  has a 65C02, ROM, RAM, VIA and ACIA just like mine.  Obviously it has the advantage that all connections are in place and should be reliable.
The clock runs at 8MHz s the board should be significantly faster than my existing breadboard version.
The second enhancement is that all necessary pins, including data bus address bus and 65C02 control pins are available on headers so are easily accessible for attaching devices / peripherals.

I ordered one immediately from Mouser in the UK and it was delivered by Fedex five days later after an epic journey from Grand Prairie, Texas.

There is a getting started project provided by WDC which demonstrates how to assemble (WDC02AS), link (WDCLN) and run a program to flash the onboard X LEDs.  The program is loaded and run using the debugger (WDCDB) which shows you the assembler source, memory variables / registers and allows you to single step or run through a program.  This is magnificent and provides a whole new world, hopefully making it much easier to develop code.

A second project  providing very detailed instructions is available from Instructables to flash an external LED attached to a VIA pin.  The executable image is loaded via TIDE (Terbium IDE) into memory and can then be run using the debugger.

These WDC tools make software development a very different experience from my DIY environment and hopefully speed up the process.  All I need to do is attach appropriate hardware to the board connectors and start development.  It looks like WDC and instructables hoped for other projects to be based on the board but I haven't seen many, there are a few references on 6502.org which I can follow up.