- Joined: 18 Sep 1999
- Posts: 498
- Location: Portland, Oregon USA
|
Rodnay to the rescue
 Posted: Tue Mar 13, 2001 3:44 am
|
According to Rodnay Zak's "Programming the Z80" the following is placed on address line A15-A8 for the various I/O instructions:
IN r, (C) : register B
IN A, im8 : register A
OUT (C), r: register B
OUT im8, A: (doesn't say)
Also, I looked through the SMS schematics again and through some old documents I have from the SMSARCH days. I noticed that there are two signals KILLSA# and KSSEL# that are referred to as JyDS# and JRead# respectively in my old docs. I wonder if the Jy stands for "joystick" and that KILLSA# could possibly be used for disabling the 315-5216, preventing it from decoding port addresses 0xFE and 0xFF. If so, then I think we've got our answer as to how external devices can use ports in the range 0xC0 through 0xFF with out colliding with the joystick ports. I have no idea what KSSEL# is for, it seems to be only connected to the VDP/PSG.
OR, some of those mystery bits (0,1 and 2) written to port 0x3E to enable the BIOS or the card/cartridge slots are control bits that disable joystick port decoding and KILLSA# is an output from the 315-5216 indicating to external devices that they should decode further I/O port bus transactions. If anyone is interested, maybe a quick test could be written to determine if clearing bits 0 and 1 in the data written to 0x3F disables joystick port access. (I'd be willing to write the test myself if someone with a dev-cart (I've got to build myself one of those things) would be willing to run it...)
--
Eric Quinn
|
