2068 Display Block Moves

This program demonstrates screen region capture and restore using a self-modifying machine code routine on the TS2068/ZX Spectrum. It offers a menu to capture one of three portions of the display file (top, middle, or bottom third) or the full screen (6192 bytes from address 16384), storing the data temporarily at address 30000. The machine code, written into addresses 65356–65367, implements a standard Z80 LDIR block-move sequence assembled via individual POKEs, with source and destination addresses computed at runtime by decomposing values into high and low bytes. A reverse LDIR pass then restores the captured region back to the display file, and the result can be saved as a CODE file named “SCRN”. The program also supports loading a pre-existing SCRN display file for comparison.

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Program Analysis

Program Structure

The program is organised into three logical phases: menu and option selection (lines 10–70), screen capture from display file to RAM buffer (lines 80–250), and RAM buffer restore back to display file with optional save (lines 260–370). Utility lines at 9997–9999 handle saving the program itself and the captured screen CODE file.

1. Lines 1–20: CLEAR 29999 reserves RAM above address 29999; menu display and keypress wait loop.
2. Lines 30–70: Dispatch on key k$; compute STP (start address), INS (address high byte), NOB (number of bytes), INB (bytes high byte) for each region option.
3. Lines 80–110: Fill the screen with a tile pattern and print row numbers 0–21 so a visible test image is present during capture.
4. Lines 120–240: POKE a Z80 LDIR routine into addresses 65356–65367, then call it with RANDOMIZE USR 65356.
5. Lines 260–370: Reverse the LDIR arguments (source and destination swapped) to restore the buffer back to screen, then offer save or menu return.
6. Lines 9997–9999: SAVE "SCRNmove" LINE 1 saves the BASIC program; SAVE "SCRN" CODE STP,NOB saves the captured screen region.

Machine Code Routine

The machine code is assembled piecemeal into high RAM (address 65356 on a 64 KB address space, i.e. wrapping into the mapped area). The routine is a minimal Z80 block copy:

Address	Opcode(s)	Mnemonic	Purpose
65356	01 lo hi	LD BC,NOB	Byte count
65359	11 lo hi	LD DE,30000	Destination (RAM buffer)
65362	21 lo hi	LD HL,STP	Source (display file)
65365	ED B0	LDIR	Block copy HL→DE, BC bytes
65367	C9	RET	Return to BASIC

For the restore phase (lines 280–340), only the byte-count and address POKEs are updated; the opcodes at 65356, 65359, 65362, 65365, and 65367 are left in place from the initial write, saving re-POKEing the opcode bytes.

Address Decomposition Idiom

Because POKE takes a single byte value, 16-bit addresses and counts are split into high and low bytes. The pattern used throughout is:

• High byte: INS = INT(STP/256)
• Low byte: STP-(256*INS) (equivalent to STP MOD 256)

This avoids the MOD operator (absent on some dialects) and is a common Sinclair BASIC technique for packing 16-bit values.

Screen Region Parameters

Key	Option	Start (STP)	Bytes (NOB)	Notes
1	Full screen	16384	6192	6144 pixels + 48 attribute bytes
2	Top third	16384	2045 (typo)	Should be 2048; “204S” is a listing error
3	Middle third	163S4+2048 (typo)	2048	“163S4” should be 16384
4	Bottom third	16384+4096	2096	Likely should be 2048; 2096 is suspicious

Bugs and Anomalies

• Line 50 — NOB=204S: The literal 204S is not a valid numeric token; this is almost certainly a transcription error for 2048 (the digit 8 rendered as S).
• Line 60 — LET STP=163S4+2048: Similarly, 163S4 is a garbled 16384 (8→S substitution).
• Line 70 — NOB=2096: A bottom-third capture should be 2048 bytes; 2096 would overrun the pixel region into attributes or beyond.
• Line 360 — INKEY*: The * should be $ (INKEY$); another transcription artifact.
• Line 330 — REM MSB sou rce: A space in a REM comment — harmless but suggests the listing was typed or OCR’d with errors.
• Line 90 — screen fill "\`.": The backtick is the £ character in Sinclair character sets; this fills the screen with £. pairs as a test pattern.
• Full-screen NOB=6192: The standard Spectrum display is 6912 bytes (6144 pixels + 768 attributes). 6192 only covers 6144 + 48 attribute bytes, leaving most attributes uncaptured. This appears intentional for a partial-attribute save but is not documented in the menu.

Notable Techniques

• CLEAR 29999 places the BASIC stack and the machine code buffer (at 30000 and 65356) safely outside the display file and program area.
• Re-using the partially-POKEd routine for both directions (capture and restore) by only overwriting the operand bytes reduces code and POKE count.
• PRINT #0;AT 1,0; writes status messages to the lower screen (stream 0) without disturbing the captured display contents in the upper screen.
• The SAVE "SCRN" CODE STP,NOB at line 9998 uses the runtime variables directly, so the correct region is always saved regardless of which menu option was chosen.