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RTM/Z80 is a multitasking kernel, built for Z80 based computers, written in Z80 assembly language, providing its users with an Application Programming Interface (API) accessible from programs written in the C language and the Z80 assembly language.

It is intended to be a simple and easy to use learning tool, for those who want to understand the tips and tricks of the multitasking software systems.


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8 Bit computers: 2022 Edition!

Due to COVID19 pandemic, I and my family was forced in our home.
So I revamped my 8bit computer book, with a new look and a nice search engine.

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Commodore 16: the misundestood

Nel 1984, dopo il successo del C/64, la Commodore lanciò sul mercato il Plus4 ed il Commdore16, due macchine quasi identiche che si differenziavano per il quantitativo di RAM a bordo e per la dotazione del software in ROM.

Il Commodore 16 idealmente doveva soppiantare il Vic20, aveva 16 KB di RAM e il Basic 3.5, notevolmente più comodo del BasicV2 della coppia Vic20, C/64.
La CPU era anche il 75% più veloce rispetto ai predecessori, ma il chip grafico era incompatibile con il C/64 e il C/16 non disponeva né di Sprite né della profondità sonora del SID:

The C16’s failure in the US market was likely due to a lack of software support, incompatibility with the C64, and lack of importance to Commodore after its competitors withdrew from the market.
Da Wikipedia

Il Plus/4, la versione potenziata del C/16 che doveva soppiantare il C/64 non scalfì neppure la superficie del suo parco applicativo e delle sue solide basi; per capirlo basta guardare al comparto dei videogame: quasi tutti i giochi erano pensati per il C/16 e quindi sfruttavano solo 16Kb di RAM, ignorando il fatto che il Plus/4 aveva la medesima memoria del C/64.

Infine la scelta di cambiare tutti gli attacchi (forse per costringere al riacquisto delle periferiche) fu una mossa infelice.

Fu un peccato perché il C/16, magari con 64kKB di RAM e le periferiche compatibili, poteva fare la differenza.

Ultimamente ci ho giochicchiato con l’emulatore VICE e se vi piace il retro computing, fateci una capatina: abbbiamo anche sistemato un problema sulla tastiera italiana, per cui su Linux Gtk l’uguale ora funziona :)

Nel frattempo il Commodore64 era duro a morire: nel 1986 la Commodore produsse una nuova versione con lo chassis bianco ma per il resto identico (il “C64C”), spia del fatto che il parco software del vecchio Commodore stava diventando un fattore cruciale per la sostenibilità del mercato.

Tra le feature più interessanti del C16/Plus4, c’è la modalità di inserimento automatico, che oramai è lo standard di tutti gli editor


Sito pieno di informazioni su Plus4/C16

Reference manual

Memory map del C16

Il sorgente del Basic TED

KernelEmu, un emulatore a riga di comando dei vari Basic Commodore, che però al momento funziona maluccio…

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C/64 Ads from 80s still have some to say on 2021?….

The Commodore 64 is a fascinating machine. It is the single best selling computer model in human history. The fact that the C64 still holds that title in 2021 — close to three decades after being discontinued — is nothing short of amazing.

Because it makes me smile, let’s take a quick stroll through some of the magazine advertisements for the Commodore 64 that helped the system achieve that world record.
See Commodore 64 ads from the 1980s still make me want a C64 in 2021


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Nice Vic20 Articles

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Turbo Rascal rulez!

Turbo Rascal is a development environment for the Commodore computers but also for others like NES and so on.

It is a bit different from other IDE because it is based on a Pascal-like language. It is not easy to compile high level programming language on 6502, but Pascal seems to pay its bill well.

What we like

  • A huge set of example (250+) and very good examples even on tiny computer like Vic20
  • A very broad support: not only Commodors 8bit computers, but also NES, Gameboy, Amiga, Atari ST, ZX Spectrum, Atari 2600, DOS and more, even some mega65 examples (!)
    A curated list of suggested emulators is provided
  • A very good list of support library, mostly for C/64:
  • Built-in ray tracer

We tested it with VICE emulator and it worked very well. Also the project is alive and its GitHub source code is updated even on these days.

What it is weird

  • The name is quite weird
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HP 42s the BEST calculator and its FAST MODE

The HP 42S accompanied me throughout my university career, secretly designed for me some function graphs and with its 7KB alphanumeric also managed to make me keep some secret notes.

I bought it on December 5, 1992, paying it 229,000 lire, a year before I started university (about 200€ now considering inflation * ).

I risked losing her towards the penultimate year of University, but the fate (and honesty of a colleague of mine) brought her back to me. It still works, and lately I have discovered that it has an internal “monitor” and the ability to set a “FAST” mode.

The HP 42S is quite easy to use, allows you to use mixed data types (e.g. matrices, complex numbers, dry text, etc.) and provides everything you need for a general-purpose engineer (numerical integrations, solvers, etc.).

There is a new implementation based on the open source Free42 (DM42) software but it costs a huge amount, more than the HP Prime which is the latest version of this generation of calculators.

The hardware design is truly refined, and the processor is able to operate natively with the required precision (and this can be seen from its computational speed).

Microprocessor: The Saturn

The HP 42S uses a chip called Saturn (at 1Mhz) that thinks internally at 4bit (nibble) but has 64bit registers (here you will find a somewhat elementary manual, poorly written but at least lists all the instructions of Saturn).

It is a chip common to all HP calculators of the time. The operating system of the HP42S (64KB ROM) is also derived from the previous calculators.


At one point (in 2003) HP implemented a Saturn emulator with an ARMv4 chip as Samsung could no longer produce it (!) with its factories. Of course, the ARMv4 also came out of production and therefore the Saturn architecture was officially abandoned around 2015, when the HP Prime was born which can be found around 150 € and still contains the RPN system with reverse Polish logic, much loved by admirers of HP calculators (myself included).


The Saturn routes 5 nibbles (20bits) and is based on 4bit serial buses.

It can then route up to 512 KB of “normal” RAM or better 1 MB of nibbles


A-D — 4 generic 64bit registers (GPR- General PuRpose registers that are sophisticatedly structured)

R0-R4 4 support registers on which calculations cannot be performed (always at 64bit)

D0, D1 two 20bit pointer registers

RTSK -> “Return STacK”, 8-position hardware stack at 20bit


The architecture is “little endian” like the x86 and 6502, so the less significant nibbles are stored before the most significant ones.

In addition, the calculator reasones using BCD logic (binary-encoded decimals) whose greatest usefulness is that you can use them without having to convert between binary and decimal and have rounding problems.

The “Saturn” is able to reason in BCD natively, and with 64bit can handle a dozen significant digits and a three-digit exponent (15 nibbles):

In this scheme, a number can have a sign (S) a mantissa of 12 digits (M) and three of exponent (X) with limit +/- 499.

GDR reigster can contain a 20-bit address (also).

Machine Language

Opcodes are variable-length (typically 2 bytes – 4 nibbles). and the architecture is vaguely log-based (you can’t do a direct RAM-to-RAM operation).

The hardware stack is an aggressive and somewhat limiting choice, but it certainly makes the Saturn faster.

Here you will find more technical details on the memory map.

As you notice, RAM takes up a 32Kb addressing space and so there are also those who have found a way to expand memory to 32Kb.

Open Source Implementations

For those who love HP42-S there is a very nice free implementation called Free42, which re-implements all the functions without emulation, and of which you will find the apps for iPhone and Android for free.

Bonus: Black jack game


  1. Wikipedia Saturn refrence (screenshot taken from there).
  2. https://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=266
  3. ROM DUMP software (a HP48 is needed :( )

(*) On eBay, it’s $200 and up.

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