02880: stactics: Scratchpad RAM is too large (currently 2 KB, should be 256 bytes with mirroring).

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
02880	Original Reference	Minor	Always	Jan 26, 2009, 07:48	Jan 29, 2009, 09:24

Tester	chowell	View Status	Public	Platform	MAMEUI
Assigned To		Resolution	Fixed	OS	Windows XP/Vista 32-bit
Status [?]	Resolved			Driver	sega/stactics.cpp
Version	0.129	Fixed in Version	0.129u3	Build	Normal
		Fixed in Git Commit		Github Pull Request #

Summary	02880: stactics: Scratchpad RAM is too large (currently 2 KB, should be 256 bytes with mirroring).
Description	The stactics.c driver currently includes the following entry in the main address map: AM_RANGE(0x4000, 0x47ff) AM_RAM This specifies 2 KB of scratchpad RAM. However, examining the schematics and parts lists in the Space Tactics manual, available at: http://www.cityofberwyn.com/manuals/SpaceTactics.pdf shows that the scratchpad RAM is implemented with only two 256 x 4-bit RAMs (AM91L11APC, AMD's version of the 2111A) designated as IC152 and IC156. (See PDF page 39.) PDF page 49 shows these RAMs, which are activated for all addresses in the address range 0x4000-0x47FF. PDF page 46 shows the address decoder (IC133) which generates this signal. Address bits A8 to A10 are ignored. Observing the game from the MAME debugger seems to further support this. Only one RAM location within 0x4100-0x47FF is accessed, but this appears to be simply a mirror of the corresponding location within 0x4000-0x40FF. There is no sign that the program wants to use more than 256 bytes of scratchpad RAM space. This problem doesn't seem to affect gameplay, since the aforementioned single location is never accessed from the 0x4000-0x40FF range, so there is no aliasing problem. But it does result in an inaccurate simulation of the hardware and is misleading to someone trying to understand the hardware from the MAME driver.
Steps To Reproduce
Additional Information
Github Commit

Flags	Verified with Original
Regression Version
Affected Sets / Systems	stactics

Attached Files