Update get_last_read_value source documentation

Machines/Apple/AppleII/Video.hpp

@@ -129,14 +129,56 @@ template <class BusHandler, bool is_iie> class Video: public VideoBase {
 		*/
 		uint8_t get_last_read_value(Cycles offset) {
 			// Rules of generation:
-			// (1)	a complete sixty-five-cycle scan line consists of sixty-five consecutive bytes of
+
-			//		display buffer memory that starts twenty-five bytes prior to the actual data to be displayed.
+			// FOR ALL MODELS IN ALL MODES:
-			// (2)	During VBL the data acts just as if it were starting a whole new frame from the beginning, but
-			//		it never finishes this pseudo-frame. After getting one third of the way through the frame (to
-			//		scan line $3F), it suddenly repeats the previous six scan lines ($3A through $3F) before aborting
-			//		to begin the next true frame.
 			//
-			// Source: Have an Apple Split by Bob Bishop; http://rich12345.tripod.com/aiivideo/softalk.html
+			//   - "Screen memory is divided into 128-byte segments. Each segment is divided into the FIRST 40, the
+			//      SECOND 40, the THIRD 40, and eight bytes of no man's memory (UNUSED 8)." (5-8*)
+			//
+			//   - "The VBL base addresses are equal to the FIRST 40 base addresses minus eight bytes using 128-byte
+			//      wraparound subtraction. Example: $400 minus $8 gives $478; not $3F8." (5-11*)
+			//
+			//   - "The memory locations scanned during HBL prior to a displayed line are the 24 bytes just below the
+			//      displayed area, using 128-byte wraparound addressing." (5-13*)
+			//
+			//   - "The first address of HBL is always addressed twice consecutively" (5-11*)
+			//
+			//   - "Memory scanned by lines 256 through 261 is identical to memory scanned by lines 250 through 255,
+			//      so those six 64-byte sections are scanned twice" (5-13*)
+
+			// FOR II AND II+ ONLY (NOT IIE OR LATER) IN TEXT/LORES MODE ONLY (NOT HIRES):
+			//
+			//   - "HBL scanned memory begins $18 bytes before display scanned memory plus $1000." (5-11*)
+			//
+			//   - "Horizontal scanning wraps around at the 128-byte segment boundaries. Example: the address scanned
+			//      before address $400 is $47F during VBL. The address scanned before $400 when VBL is false is
+			//      $147F." (5-11*)
+			//
+			//   - "the memory scanned during HBL is completely separate from the memory scanned during HBL´." (5-11*)
+			//
+			//   - "HBL scanned memory is in an area normally taken up by Applesoft programs or Integer BASIC
+			//      variables" (5-37*)
+			//
+			//   -  Figure 5.17  Screen Memory Scanning (5-37*)
+
+			// FOR IIE AND LATER IN ALL MODES AND II AND II+ IN HIRES MODE:
+			//
+			//   - "HBL scanned memory begins $18 bytes before display scanned memory." (5-10**)
+			//
+			//   - "Horizontal scanning wraps around at the 128-byte segment boundaries. Example: the address scanned
+			//      before address $400 is $47F." (5-11**)
+			//
+			//   - "during HBL, the memory locations that are scanned are in the displayed memory area." (5-13*)
+			//
+			//   - "Programs written for the Apple II may well not perform correctly on the Apple IIe because of
+			//      differences in scanning during HBL. In the Apple II, HBL scanned memory was separate from other
+			//      display memory in TEXT/LORES scanning. In the Apple IIe, HBL scanned memory overlaps other scanned
+			//      memory in TEXT/LORES scanning in similar fashion to HIRES scanning." (5-43**)
+			//
+			//   -  Figure 5.17  Display Memory Scanning (5-41**)
+
+			// Source: *  Understanding the Apple II by Jim Sather
+			// Source: ** Understanding the Apple IIe by Jim Sather
 
 			// Determine column at offset.
 			int mapped_column = column_ + int(offset.as_integral());