Grafix

This program is a full-screen text and block-graphic editor that stores a 704-character (22×32) canvas in the string variable Z$(704). It supports cursor movement, character placement, SAVE/LOAD of the canvas to a named file, RAMTOP-based POKE/PEEK transfer to address 32000, horizontal and vertical scrolling, a mirror/flip function, find-and-replace for graphic characters, and a configurable print routine using direct POKEs to system variables around address 16529. Navigation and command dispatch are handled by a computed GOSUB (line 210: GOSUB X*30+1000), routing keypress codes to the appropriate handler subroutine. The keypress loop at lines 8900–8902 uses PAUSE 40000 followed by CODE INKEY$ as an efficient non-blocking wait.

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Program Analysis

Program Structure

The program is organised as a collection of subroutines scattered across a wide line-number range, with a main loop at lines 200–590 and a dispatch table implemented via a computed GOSUB. Lines 1–11 hold short utility subroutines that are called frequently. The initialisation block (lines 100–130) sets up the 704-character canvas string Z$, the cursor position A, the working string X$, and the mode flag Z.

Canvas Layout

The editing canvas is a 704-character string (DIM Z$(704), line 100), representing a 22-row × 32-column display. Position A is a linear index into this string. Row and column are recovered with integer arithmetic:

• Row: B = INT((A-1)/32) (line 520)
• Column: A - B*32 - 1 (line 540)

The status line at row 21 is used for prompts and is not part of the canvas data.

Computed GOSUB Dispatch

The central command-dispatch mechanism at line 210 is:

GOSUB X*30+1000

where X is the raw CODE of the last keypress. Each key maps to a handler whose line number equals X*30+1000. For example, key code 119 (the clear/home key) maps to line 119*30+1000 = 4570; the target lines 4538–4540 handle reset. This avoids a long IF/THEN chain and is a well-known BASIC optimisation technique.

Key Handler Subroutines

Key Code	Target Line	Function
112 (cursor left)	4360	Move cursor left one character
113 (cursor right)	4390	Move cursor right one character
114 (cursor down)	4420	Move cursor down one row
115 (cursor up)	4450	Move cursor up one row
116 (EXTEND)	8800	INPUT a string into X$
119 (clear)	4538	Reset X and X$
216 (SHIFT+Q)	—	Toggle insert/overwrite mode via Z
226 (SHIFT+S)	730	SAVE canvas to named file
227 (SHIFT+A)	—	STOP (exit program)

Keypress Wait Idiom

Lines 8900–8902 implement the standard efficient keypress wait:

PAUSE 40000 / POKE 16437,255 / LET X=CODE INKEY$

The POKE 16437,255 resets the FRAMES system variable so the PAUSE does not expire prematurely on repeated calls. This yields a responsive input loop without busy-waiting.

Mode Flag Z

Z tracks the editor’s insert/cursor mode. Key 216 (SHIFT+Q) toggles it: first press sets Z=1 (line 240), second press sets Z=2 (line 230), and line 550 resets it to Z=1 after each frame. When Z=1, the current X$ is displayed at the cursor position (line 540) without being committed to Z$; when Z<>1 and X$>"", X$ is written into Z$ (line 490).

Direct Memory Access

Several routines use direct POKEs to system variable addresses:

• POKE 16529,X — sets the current output stream or print channel character
• POKE 16563,X and POKE 16583,X — configure character codes for the MIX PRINT and PRINT routines respectively
• POKE 16597,B — stores the file name character for the file-name routine
• POKE 16614,CODE X$(J) — clears references by character code in the REFS routine

The RAMTOP check at line 7580 verifies PEEK 16388 + 256*PEEK 16389 = 32000 before reading or writing the canvas to/from raw memory at address 32000. If the check fails, the program halts with a message (line 7590–7595).

Scroll and Mirror Functions

Lines 7690–7719 implement four-directional scrolling by slicing and concatenating Z$:

• Scroll up (A=68): Z$(33 TO)+Z$(1 TO 32)
• Scroll down (A=132): Z$(673 TO)+Z$(1 TO 672)
• Scroll left (A=99): Z$(2 TO)+Z$(1)
• Scroll right (A=101): Z$(704)+Z$(1 TO 703)

Lines 7720–7739 implement a horizontal mirror of the canvas. The routine reverses character order within each row (stepping J from 32 to 1) and uses a lookup string X$ to translate asymmetric block graphics to their mirrored equivalents, ensuring that graphical characters remain visually correct after reflection.

Inverse-Video Toggle

Lines 7510–7539 invert every character in Z$ by toggling bit 7 of its character code: characters below 128 gain 128, characters above 127 lose 128 (with a special case for code 192, mapped to 267−128=139 to avoid an out-of-range CHR$). After the pass, the routine resets X to 119.

Find and Replace

Lines 7630–7649 implement a simple single-character find-and-replace across the entire canvas: the user inputs the graphic to be replaced (X$) and the replacement (Y$), then a FOR loop substitutes every matching cell in Z$.

Notable Anomalies

• Line 7529 uses GOTO 7536 to skip the second branch of the inversion loop, but line 7525 contains NEXT J and line 7535 also contains NEXT J: the loop at 7514 has two NEXT J statements on different branches, which means the FOR loop at 7512 is only iterated correctly on the first pass through the non-inverse branch; control then falls through or jumps to line 7536. This is a structural oddity that may cause premature loop termination in the inverse branch.
• The subroutine at line 4480 is a bare GOTO 8700, delegating entirely to the graphics-input handler without any wrapping logic.
• Line 7526 returns immediately if Y$="" after the co-ordinate input, but the co-ordinate encoding at line 4528 assumes exactly two characters in Y$ without bounds checking.