qsnesdoc.html

SNES game emulator. C and asm files.

windowing, the appropriate bits in <A HREF="#Reg212E">registers $212E and
$212F</A> must be set in addition to the bits in these registers.

<P><A NAME="Reg2125"></A><B><FONT FACE="Lucida Console">Register $2125:
Window mask settings #2 (1b/W)</FONT></B>

<P><TT>ccccssss&nbsp; c: Settings for "color windows"&nbsp; s: Settings
for sprite windows</TT>

<P><TT>Both of these sets of four bits are formatted the same as <A HREF="#Reg2123">registers
$2123 and $2124</A>.</TT>

<P>This is like the last set of registers except it is for sprites and
"color windows".&nbsp; I don't know what color windows are; won't someone
explain it?

<P><A NAME="Reg2126"></A><B><FONT FACE="Lucida Console">Register $2126
to $2129: Window position designations (4*1b/W)</FONT></B>

<P><TT>$2126&nbsp; Window 1 left&nbsp; position</TT>
<BR><TT>$2127&nbsp; Window 1 right position</TT>
<BR><TT>$2128&nbsp; Window 2 left&nbsp; position</TT>
<BR><TT>$2129&nbsp; Window 2 right position</TT>

<P><TT>For each of these bytes:</TT>
<BR><TT>xxxxxxxx&nbsp; x: pixel position of window left or right position.</TT>

<P>These regs simply tell the range of the window.&nbsp; The window range
is inclusive, so if clipping is IN, and the window range is from pixels
3 to 6, all four pixels will be clipped rather than just pixels 4 and 5.&nbsp;
If clipping is out, however, clipping will be from pixels 0 to 2 and 7
to 255 inclusive.

<P><A NAME="Reg212A"></A><B><FONT FACE="Lucida Console">Register $212A
and $212B: Mask Logic settings (2*1B/W)</FONT></B>

<P><TT>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
Parameters for:</TT>
<BR><TT>$212A&nbsp; 44332211&nbsp; 4: BG4&nbsp; 3: BG3&nbsp; 2: BG2&nbsp;
1: BG1</TT>
<BR><TT>$212B&nbsp; ????ccss&nbsp; c: Color windows&nbsp;&nbsp;&nbsp; s:
Sprites</TT>

<P><TT>Each of the two bits for each BG and sprites determines the mask
logic as follows:</TT>

<P><TT>00&nbsp; OR&nbsp;&nbsp;&nbsp;&nbsp; 10 XOR</TT>
<BR><TT>01&nbsp; AND&nbsp;&nbsp;&nbsp; 11 XNOR</TT>

<P>Apparently, these registers apply only for BGs and sprites that have
both windows enabled at the same time.&nbsp; If only one window is enabled,
these bits should be ignored.&nbsp; I'm not sure whether the logic is applied
to the visible regions or the clipped regions.&nbsp; For example, if pixels
10-20 and 50-60 are clipped, I don't know whether OR or AND would be used
to clip both regions and which would clip neither.

<P><A NAME="Reg212E"></A><B><FONT FACE="Lucida Console">Register $212E
and $212F: Window designation (2*1b/W)</FONT></B>

<P><TT>$212E&nbsp; Main-screen window designation</TT>
<BR><TT>$212F&nbsp;&nbsp; Sub-screen window designation</TT>
<BR><TT>For each of these:</TT>
<BR><TT>---sdcba&nbsp; s: sprites enable&nbsp; d: BG4 enable&nbsp; c: BG3
enable</TT>
<BR><TT>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
b: BG2 enable&nbsp; a: BG1 enable</TT>

<P>These registers are formatted the same as the visibility designation
<A HREF="#Reg212C">registers $212C and $212D</A>.&nbsp; A bit must be set
in both $212C and $212E or $212D or $212E in order for windows to be enabled
for that layer (BG or sprites).
<BR>&nbsp;
<CENTER><A NAME="RegCounter"></A><FONT SIZE=+2>Counter/IRQ/NMI Registers</FONT></CENTER>

<CENTER>&nbsp;</CENTER>


<P><A NAME="Reg2137"></A><B><FONT FACE="Lucida Console">Register $2137:
Software latch for h/v counter (1b/R)</FONT></B>

<P>This register does not return a useful value.&nbsp; Apparently, though,
you have to read from this register before attempting to read $213C or
$213D.

<P><A NAME="Reg213C"></A><B><FONT FACE="Lucida Console">Register $213C
&amp; $213D: Horizontal/Vertical scan location (Db/R)</FONT></B>
<PRE>For each of the registers:
-------l llllllll&nbsp; l: scanline number ($213C) or horizontal position ($213D)
* vertical range is 0 to 261, 0=top of screen (values over 0-223/0-238 are in the vblank)
* horizontal range is from 0 to 339, 0=far-left of screen.</PRE>
This register tells where exactly on the screen the SNES is sending data
to the TV. For the horizontal counter, apparently, zSNES always alternates
between sending a strange value (most often 8) and 1, respectively, in
register $213C, but only after $2137 has been read once.&nbsp; For the
vertical counter, the low byte of the scanline is sent, followed by the
high byte, and afterwards nothing but zeros (until $2137 is read again.)&nbsp;
One doc suggests register $213F is related to this register, but I don't
know how so.

<P><A NAME="Reg4200"></A><B><FONT FACE="Lucida Console">Register $4200:
Counter Enable (1b/W)</FONT></B>
<PRE>n-vh---j&nbsp; n: NMI Interrupt enable&nbsp; v: vertical counter enable
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; h: horizontal counter enable&nbsp; j: joypad enable</PRE>
n and j must be set to enable NMI interrupts and the joypad.&nbsp; Not
sure of the exact effects of bits 4 and 5, but they probably must be set
to enable IRQ interrupts.

<P><A NAME="Reg4207"></A><A NAME="Reg4209"></A><B><FONT FACE="Lucida Console">Register
$4207/$4208 &amp; $4209/$420A: Horizontal/Vertical IRQ trigger counters
(2*2b/W)</FONT></B>
<PRE>For each register:
???????l llllllll&nbsp; l: location to trigger IRQ</PRE>
When IRQs are enabled and the PPU starts drawing at the specified location,
an IRQ is generated.&nbsp; Horizontal IRQs are rarely used, because of
the high frequency (every scanline) of the interrupts.

<P><A NAME="Reg4210"></A><B><FONT FACE="Lucida Console">Register $4210:
NMI Register (1b/R)</FONT></B>
<PRE>n---vvvv&nbsp; n: nmi flag&nbsp; v: version number</PRE>
The full behavior of these bits is not known (at least, I don't know).&nbsp;
Yoshi doc lists "$5A22" for the v bits, whatever that means.&nbsp; Mario
World reads this register at the beginning of its NMI procedure and then
discards the value read.&nbsp; zSNES apparently returns a 1 in the n bit
when NMIs are enabled AND a vblank is in progress.

<P><A NAME="Reg4211"></A><B><FONT FACE="Lucida Console">Register $4211:
IRQ Register (1b/RW?)</FONT></B>
<PRE>i-------&nbsp; i: irq flag</PRE>
The full behavior of these bits is not known (at least, I don't know),
so take this description with a big grain of salt.&nbsp; If bits 4/5 of
<A HREF="#Reg4200">register $4200</A> are set, then as soon as the H/V
counter timer becomes the count value set in <A HREF="#Reg4207">registers
$4207 to $420A</A>, then an IRQ will be "applied" (unless the CPU interrupt
flag is enabled.&nbsp; If this is the case, then the interrupt will be
called as soon as the flag is cleared.)&nbsp; The IRQ routine must immediately
use SEI to prevent recursive calling, then read from this register to "un-apply"
the interrupt.&nbsp; The first read will return 1 in the i bit, then subsequent
reads will return 0.&nbsp; What happens to this register when bits 4/5
of <A HREF="#Reg4200">register $4200</A> are <I>not</I> set is unclear.
Some SNES programs use a loop to read this register (in emu's, often getting
stuck in this loop) but why a loop is used is unknown.

<P><A NAME="Reg4212"></A><B><FONT FACE="Lucida Console">Register $4212:
Status register IRQ Register (1b/RW?)</FONT></B>
<PRE>vh-----j&nbsp; v: whether in vblank state&nbsp; h: whether in hblank state
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; j: whether joypad is ready to be read</PRE>
This register is pretty simple; it returns whether in a vertical/horizontal
blank period (1=yes) and whether the joypad is ready to be read from registers
$4218 to $421F.&nbsp; The horizontal blank time lasts from "pixels" 256
to 339. (Not really pixels, since nothing is being drawn.)&nbsp; In zSNES,
<I>apparently</I>, the joypad becomes ready to be read once every frame,
and more specifically, scanline 227 (might not be a constant.)&nbsp; How
this bit gets cleared to 0, however, is unknown.

<P>A loop in the "leftover" demo suggests that the joypad bit is backwards;
that is, a value of 1 indicates the joypad is <I>not</I> ready.&nbsp; I
think some other demos suggest the opposite is true, though.&nbsp; Interestingly
enough, the demos still work regardless of their treatment of the bit.&nbsp;
Hmmm...
<CENTER>&nbsp;</CENTER>

<CENTER><A NAME="RegJoypad"></A><FONT SIZE=+2>Joypad Registers</FONT></CENTER>


<P><A NAME="Reg4218"></A><B><FONT FACE="Lucida Console">Register $4218
to $421F: Joypad registers(2*1b/R)</FONT></B>

<P><TT>$4218/$4219&nbsp; Joypad 1 data</TT>
<BR><TT>$421A/$421B&nbsp; Joypad 2 data</TT>
<BR><TT>$421C/$421D&nbsp; Joypad 3 data</TT>
<BR><TT>$421E/$421F&nbsp; Joypad 4 data</TT>

<P><TT>The two registers for each joypad contain the following:</TT>

<P><TT>Low Byte&nbsp;&nbsp;&nbsp;&nbsp; B: B button&nbsp; Y: Y button&nbsp;
S: Select button T: Start button</TT>
<BR><TT>BYSTudlr&nbsp;&nbsp;&nbsp;&nbsp; u: Up&nbsp; d: Down&nbsp; l: Left&nbsp;
r: Right</TT>
<BR><TT>High Byte&nbsp;&nbsp;&nbsp; A: A button&nbsp; X: X button&nbsp;
L: Top-left button R: Top-right button</TT>
<BR><TT>AXLR????</TT>

<P>This is the easy way to read the joypad.&nbsp; Simply set bit 0 of <A HREF="#Reg4200">register
$4200</A>, and the SNES will automatically read the state of all the buttons
(probably once every frame) into these registers.&nbsp; These registers
may be read more than once; that is, reading these registers does not destroy
their values.&nbsp; It is best to wait for bit 0 of <A HREF="#Reg4212">register
$4212</A> to be set (set or cleared?&nbsp; I'm not sure) before reading
from these registers.

<P>Although there are only two joypad ports, two extra joypads can be accessed
through $421C-$421F using a multitap device.

<P><A NAME="Reg4016"></A><B><FONT FACE="Lucida Console">Registers $4016/$4217:
Old-style Joypad registers (2*1B/R/W)</FONT></B>

<P><TT>On read:&nbsp;&nbsp; $4016 is for Player 1; $4017 is for Player
2.</TT>
<BR><TT>???????b&nbsp;&nbsp; b: button state</TT>
<BR><TT>On write:</TT>
<BR><TT>????????&nbsp;&nbsp; ?: strobe activation value</TT>

<P>These are called the "old-style" joypad registers because they are used
like the NES used them.&nbsp; First of all, to use these registers, bit
0 of $4200 must be clear (I suppose to prevent conflict between the SNES's
automatic read mechanism and the manual method.)&nbsp; If this bit is set,
then these registers simply return whether the joypad is connected (0=Not
connected.)&nbsp; Based on what commercial games do, it may be neccessary
to write a 0 before this function works.

<P>To use the old-style reading method, bit 0 of register $4200 must be
clear (probably to prevent conflict between the manual reading and the
SNES' automatic reading.)&nbsp; Like on the NES, two values should be written
before reading starts: a 1 followed by a 0.&nbsp; This "activates a strobe
in the joypad circuitry."&nbsp; Apparently, but not for certain, it is
only necessary to do this with $4016; that is, writing to one register
resets both joypads. Once the writing is done, each read from each register
will return the state of one button, in the following order: