SN76489 TestRom - Homebrew

Tool for SG-1000

Version	Date	By	Download
1.00	27 Mar 2024	JoppyFurr	SN76489TestRom-SG-1.00.zip (148KB) (includes source)

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Competition Entry

This was a competition entry in 2024

Seeing as my first entry is for testing a somewhat uncommon piece of hardware, I figured I'd make a second entry, for the SN76489/compatible sound chip available in all Master Systems.

This tool is intended to help testing real hardware against emulators to help spot differences in behaviour.

Seeing as the SN76489 doesn't have a key-on register, the volume of each channel is held at 15 (silent) until a button/key is pressed. The 'const' mode allows the volume to be passed to the chip without a key/button being pressed.

Both PAL and NTSC versions of the roms are generated, the only difference being the frequency values associated with the keyboard notes, as the clock frequency is slightly different.

Because the SG-1000 / SC-3000 use different versions of the chip compared to the Master System, I have made builds of this ROM for them, using Mode-0 to draw to the graphics.

With flash carts being less common for the SG-1000 / SC-3000, the Mode-0 build is also available as a .wav file that can be played into the SC-3000's cassette port using a BASIC IIIa or BASIC IIIb cartridge. The following steps are needed to run this over the cassette interface: 1. Run the command `LOAD` at the BASIC prompt 2. Play the .wav file into the cassette port. You may need to adjust the volume if the filename isn't detected after the first ~5 seconds. 3. Run the command `CALL &H9800` at the BASIC prompt

To get the compiled code running over the cassette port, a few changes were made to crt0_sg and SGlib, to change the memory layout and remove the reliance on interrupts.