Add STAT phrasing to clarify and emphasize the interrupt they are associated with (#280)

Author: bbbbbr

content/Video_Display.md

@@ -7,12 +7,12 @@ A *dot* is the shortest period over which the PPU can output one pixel: is it eq
 ### FF41 - STAT (LCD Status) (R/W)
 
 ```
-Bit 6 - LYC=LY Interrupt             (1=Enable) (Read/Write)
-Bit 5 - Mode 2 OAM Interrupt         (1=Enable) (Read/Write)
-Bit 4 - Mode 1 VBlank Interrupt      (1=Enable) (Read/Write)
-Bit 3 - Mode 0 HBlank Interrupt      (1=Enable) (Read/Write)
-Bit 2 - LYC=LY Flag      (0=Different, 1=Equal) (Read Only)
-Bit 1-0 - Mode Flag       (Mode 0-3, see below) (Read Only)
+Bit 6 - LYC=LY STAT Interrupt source         (1=Enable) (Read/Write)
+Bit 5 - Mode 2 OAM STAT Interrupt source     (1=Enable) (Read/Write)
+Bit 4 - Mode 1 VBlank STAT Interrupt source  (1=Enable) (Read/Write)
+Bit 3 - Mode 0 HBlank STAT Interrupt source  (1=Enable) (Read/Write)
+Bit 2 - LYC=LY Flag                          (0=Different, 1=Equal) (Read Only)
+Bit 1-0 - Mode Flag                          (Mode 0-3, see below) (Read Only)
           0: In HBlank
           1: In VBlank
           2: Searching OAM
@@ -92,24 +92,36 @@ milliseconds.
 
 ### INT 48 - STAT Interrupt
 
-There are various reasons for this interrupt to occur as described by
-the STAT register (\$FF41). One very popular reason is to indicate to
-the user when the video hardware is about to redraw a given LCD line.
-This can be useful for dynamically controlling the SCX/SCY registers
-($FF43/$FF42) to perform special video effects.
+There are various sources which can trigger this interrupt to occur as
+described by the [STAT register (\$FF41)](#ff41-stat-lcd-status-r-w).
+One very popular use is to indicate to the user when the video 
+hardware is about to redraw a given LCD line. This can be useful for 
+dynamically controlling the SCX/SCY registers ($FF43/$FF42) to [perform 
+special video effects](https://github.com/BlitterObjectBob/DeadCScroll).
 
 Example application: set LYC to WY, enable LY=LYC interrupt, and have
 the handler disable sprites. This can be used if you use the window for
 a text box (at the bottom of the screen), and you want sprites to be
 hidden by the text box.
 
+The various STAT interrupt sources (modes 0-2 and LYC=LY) have their 
+state (inactive/low and active/high) logically ORed into a shared 
+STAT interrupt line if their respective enable bit is turned on.
+
+A STAT interrupt will be triggered by a rising edge (transition from 
+low-to-high) on the STAT interrupt line.
+
 ::: warning
-As mentioned in the description of the STAT register, the PPU cycles
-through the different modes in a fixed order. If we set the STAT bits
-in a way that they would interrupt the CPU at two
-consecutive modes, then the second interrupt will not trigger. So for example,
-if we enable the interrupts for Mode 0 and Mode 1,
-the Mode 1 interrupt will not trigger.
+If a STAT interrupt source logically ORs the interrupt line high while 
+(or immediately after) it's already set high by another source, then 
+there will be no low-to-high transition and so no interrupt will occur. 
+This phenomenon is known as "STAT blocking" ([test ROM example](https://github.com/Gekkio/mooneye-gb/blob/2d52008228557f9e713545e702d5b7aa233d09bb/tests/acceptance/ppu/stat_irq_blocking.s#L21-L22)).
+
+As mentioned in the description of the [STAT register](#ff41-stat-lcd-status-r-w), 
+the PPU cycles through the different modes in a fixed order. So for 
+example, if interrupts are enabled for two consecutive modes such as 
+Mode 0 and Mode 1, then no interrupt will trigger for Mode 1 (since 
+the STAT interrupt line won't have a chance to go low between them).
 :::
 
 # LCD Position and Scrolling