1 - Tape Loaders

1.1 - TAP

A .TAP file is a simple image format used to represent data saved to tape. It's a simple image format which can be generated by some assemblers like zasm.

Now we have a simple BASIC loader which generates a .TAP file. This is effectively a simple BASIC program which when run loads the next file on the tape into memory at address 24000 0x5DC0 and then executes the machine code at that same address.

Hello World Example

For this example we will write a simple machine code program which writes Hello World to the screen.

1wget https://area51.dev/sinclair/asm/loaders/tap/loader.tap
2wget https://area51.dev/sinclair/asm/loaders/tap/helloworld.z80
3zasm helloworld.z80
4cat loader.tap helloworld.tap > tape.tap

The commands do the following:

1. Download loader.tap which is the precompiled loader. The source is viewable here along with how to compile it yourself.
2. Download helloworld.z80 which is the source shown below.
3. Compiles the source generating helloworld.tap.
4. Concatenates both tap files to generate our final tape.tap file.

Links to the required files are available in the Resources panel at the top right of this page including a precompiled helloworld.tap file.

Running the example

If no errors occurred you can run it with the fuse emulator:

1fuse tape.tap

You should then see something like this screenshot.

The first line visible is from the boot loader as it loaded the file in helloworld.tap.

The second line is the output of the source below.

helloworld.z80 source

1.1.1 - BASIC Loader

This shows the source for the BASIC .TAP loader for use with the zasm assembler.

To compile run the following commands in a Linux shell to download the sources and compile the loader:

1wget https://area51.dev/sinclair/spectrum/reference/include/zasm/headers.z80
2wget https://area51.dev/sinclair/asm/loaders/tap/loader.z80
3zasm loader.z80

The commands do the following:

1. Download headers.z80 which contains the definitions for the Spectrum BASIC tokens we require.
2. Download loader.z80 which is the source shown below.
3. Compiles our loader generating loader.tap.

The generated loader.tap can now be used with your own .TAP file as long as it loads at address 24000 and its entry point is also at that same address.

All you need to do is concatenate both .TAP files with loader.tap first. The Hello World example shows how this is done.

Links to the required files are shown in the resources panel at the top right of this page.

2 - Managing the Screen

2.1 - Clear the Screen

This code snippet will clear the standard screen on all versions of the Spectrum.

It's broken down into three sections:

1. Clear's the pixel data
2. Reset the screen's colour data, in this instance to White text on Black Background
3. Sets the screen border to black

Important note here:

We copy 6144 bytes in the first LDIR not 6143 which we would normally do if we want to erase just the DISPLAYFILE.

Using 6144 bytes will cause the first byte of DISPLAYATTR to be set to 0 which is fine here as we want HL and DE to be pointing to DISPLAYATTR ready for the second LDIR operation.

This could, in theory cause the top left character to flicker briefly as it's been set to 0 (BLACK foreground and background). If that's an issue then set the BC to 6143 and increment HL and DE after the LDIR as in the next example:

There are faster but longer methods of clearing the screen which I won't go into here. For most purposes the first example above will work for most purposes other than time critical games.

2.2 - Get address of character on screen

Calculating the start address for a character might sound complicated due to its weird memory layout, but it is actually pretty simple.

The way the address is calculated is described with the Display File, however the following code allows you to calculate the address in Z80 machine code.

A link to download this routine is available under Resources at the top right of the page:

Note: This code is actually from my Teletext Mode 7 emulator for the Spectrum

2.3 - Set the border colour

The border colour is managed by the ULA. Changing the border colour is simply writing the required colour to the ULA whilst storing a copy in the BORDCR system variable at 0x5C48 so that the OS also knows what the colour is.

In this example we set the Border to black

3 - Determine Machine Type

There is no official method of determining which model of Spectrum code is running on, however one method is to look for differences in the ROM on the machine.

There were six official models of Spectrum released:

1. 48K with a single rom (16K had the same ROM)
2. 128K with 2 roms
3. Spanish 128K released 1985
4. +2 with a built-in Cassette Deck
5. +2A and +3 - These have the same roms just differ with the +3 having a built-in 3" floppy drive instead of the cassette deck.

Below is a routine which will return a value representing the machine type the code is running on in the Accumulator.

It works by first paging out the 48K rom - something that will do nothing on a real 48K machine, before checking for the Copyright string present in the 48K rom.

If that is not present it then attempts to page out the rom on a +2A/+3 machine - as they have 4 ROM's, then checking for strings present on the +2A/+3, then the +2 and finally the Spanish 128K machine.

If it got this far without a match then it's running on an original UK 128K machine.

Rom selection

There's two additional routines in here, romSel1 and romSel2, which handle the ROM selection and could be reused. They both take the new value of the port which manages the rom selection in the Accumulator and updates both the appropriate port and system variable.

For romSel1 load the Accumulator from 0x5BFC and make your changes before calling this routine.

For romSel2 load the Accumulator from 0x5B67 and make your changes before calling this routine.

machinetype.z80 source