HP82143A printing - two views of the same medal

[brouhaha]

(07-17-2024 02:10 AM)Christoph Giesselink Wrote:  Here we have a character buffer from address 0x14 to 0x3f which are 44 byte, but it seem to be implemented as ring buffer with a "current" and an "end" pointer. A ring buffer regularly can only hold "buffersize - 1" elements because when Read and Write pointer are equal, this means empty buffer.

That property of a circular buffer is true only if there is not some independent mechanism for tracking full vs empty, using at least one additional variable. Such a variable could be a single-bit flag or a buffer fill count.

For instance, fi the buffer pointers are equal, and the last buffer operation was a read, then the buffer is empty. If the buffer pointers are equal, and the last buffer operation was a write, then the buffer is full.

I suspect that the 82143A firmware does maintain additional state to distinguish full from empty, but I haven't specifically identified the details.