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Update get_last_read_value source documentation

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Ryan Schmidt 2023-10-24 19:50:02 -05:00
parent c206c7e2cb
commit 18ed36d090
1 changed files with 49 additions and 7 deletions
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56
Machines/Apple/AppleII/Video.hpp
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@ -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.
// (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.
// FOR ALL MODELS IN ALL MODES:
//
// 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());