Sprite fetch effective address incorrect

[hernandp]

Test Environment (required)

Describe the bug (Run in 64 mode)

• Setup MEGA65 I/O Mode.
• Set screen pointer to 0x008C00
• Set sprite table pointer to 0x008C00 + 0x3F8
• Set sprite 0,1,2 address to 32 (32 * 64 = 0x800)

According to documentation:

• If default bank is 3 (base address $0000) and VICII banking is considered in sprite fetch effective address, Sprite 0 will be read from $800.
• If VICII banking is ignored, and SPRPTR16 is not used, I expected address fetch to be also from $800

But copying sprite data to $8800 will display the sprite 0 properly. This is I think, Incorrect (or undocumented) behavior.

To Reproduce

Use the attached C program to play, or use the provided BANK65.PRG.

bank65.zip

Expected behavior Sprite 0 must be displayed as garbage from $800.

[gardners]

The sprite data address always comes from the same VIC-II bank as the sprite pointer address, according to the VHDL (see around line 4980 of viciv.vhdl). What does the documentation say?

[hernandp]

Documentation just says: ". This is done by setting the SPRPTR16 signal (bit 7, $D06E, decimal 53358), which tells the VIC-IV to expect two bytes per sprite pointer instead of one. These addresses are then absolute addresses, and ignore the 16KB VIC-II bank selection logic. "

That's quite understandable: SPRPTR16 will make VICII bank to not affect sprite fetch address.

But how address generation works when SPRPTR16 is OFF, the sprite pointer register selects e.g: to put the table at $50000 and VIC-II banking is active. What I found counterintuitive is why the sprite pointer table should affect the sprite pointers themselves, as the documentation states the $D06C - $D06E is just for selecting the table location in memory, not the addresses of the sprites.

In other words, if I have the default pointer table, with default bank ($DD00=3) at screen+$3F8 with the first entry set at 13 that yields address 832 for sprite #0. Thats correct I guess.

But if I set $D06E=$05, $D06D=$00, $D06E=$00 to set the pointer table at $50000, first entry to value 13, and I'm still with default VICII bank ($DD00=3), why it wont fetch also from 832 ? That's not stated in documentation.

I hope that I'm clear with my doubts.
Regards, hernan

[hernandp]

@gardners I understood. Bits 14 / 15 can be set by the Sprite pointer VICIV registers $D06C..0E or by $DD00 if HOTREGS are enabled. My confusion was believing that $DD00 was the unique responsible -always- to set banking bits (14-15). I think that is can be closed. We could review documentation to see if this is stated clear.
XEMU fix coming along. Thank you!.