ASM Diaries 4: Don’t Optimise Error Paths
This piece should really be called ‘optimise the path which is not the error path’ but that’s a very unsexy title. The point here is that your code’s error path is (usually) executed a single time before returning to the user for input. That says that it’s not time-sensitive and you don’t need to optimise …. Read More
ASM Diaries 3: Eliminate Jumps by Pre-loading
Writing the code to validate ranges in type definitions in Quiche-Z80 I needed to test for some values. I could easily handle the cases where both values where unsigned or both values where signed but the case where one was signed and the other unsigned allowed a nice little optimisation. Due to the typing rules …. Read More
ASM Diaries 2: A Hack for Case Insensitive Identifiers
This article will make a lot more sense with an ASCII table to look at. I don’t have quick access to a copyright free one, but you view one at https://www.asciitable.com Case insensitive comparisons in assembler are hard work. The design of the ASCII codes splits letters into two blocks. That requires your code to …. Read More
ASM Diaries 1: Type Identifier Equals Data Length
A few years ago I reverse engineered Amstrad CPC BASIC. I’m currently writing a compiler for my Pascal-like language (called Quiche). This is an occasional series of articles where I note observations about assembly programming and, in particular, Z80 assembly language. This is a trick I learnt while reverse engineering the Amstrad BASIC interpreter. The …. Read More
Why Your Amstrad CPC Might Crash in 2104!
Recently I was reading the source code of the firmware routine which updates the system TIME in the Amstrad CPC. This code contains a bug which will eventually crash your machine! Read on for the full gory details! Updating the Timer The TIME counter is a is 32-bit wide integer (four bytes) which is updated …. Read More
Writing a Compiler 1: Expressions and Intermediate Language
Recently I’ve been getting increasingly broody to write a compiler. They’ve always been something of a mystery to me and, while I’ve always fancied writing one, I’ve never had a convincing reason to do so. But, since I’ve returned to retro computing, I can see definite limitations in the options currently available for writing software …. Read More
Passing Code Pointers as Data in Amstrad CPC BASIC
One of my secret coding pleasures is passing a function as a parameter to a subroutine. Most modern languages have what’s called ‘first class code’. That means that you can assign the address of a function to a variable, store it in an array, and pass it as a parameter to a function. This enables …. Read More
How Amstrad CPC BASIC Compresses Error Messages
Amstrad CPC BASIC saves a few bytes by using some interesting compression in the way it stores error messages. Let’s take a look at how it works and how much space it saves. Below is the table of error messages. The messages are stored in what I refer to as ASCII7 format – bit 7 …. Read More
Understanding the Amstrad CPC Video, RAM and Gate Array Subsystem
The Amstrad CPC is my favourite 8-bit home computer system – I still have my childhood CPC464. There are a number of things I love about it’s design, including the video system, which offers higher resolution and a larger colour palette than most of it’s siblings. Recently I’ve put some effort into understanding how the …. Read More
Couch to 64k Part 5: Adding RAM and a CP/M Compatible Memory Architecture to the Z80 Breadboard Computer
If you’ve been following this series so far then you’ll have a basic Z80 based computer with ROM, an LCD display and a keypad or keyboard. What we don’t have yet is any way store store data. We need to add some RAM (Random Access Memory). This is what I’ll be focusing on in this …. Read More