Alignment

This program generates five TV and monitor alignment test patterns on a ZX Spectrum: color bars, horizontal lines, vertical lines, a crosshatch grid, and a dot grid. The color bars routine (lines 1000–1550) uses PAPER attributes to paint eight 4-column-wide strips across the screen, labeling each with a three-character color abbreviation stored in a DIM c$(8,3) array. The crosshatch pattern reuses the horizontal-lines subroutine (line 2000) and then falls through to draw vertical lines starting at line 3010, sharing code between options 2 and 4. A border-drawing subroutine at line 6000 uses four DRAW commands to trace a rectangle around the screen edge. The menu returns control by branching to line 1 (a non-existent line), which causes the program to restart from the top.

Program Structure

The program is organized into a menu section and five pattern-generating blocks, each anchored at a round thousand line number. A setup subroutine at line 6000 draws a screen border rectangle. Navigation from any pattern back to the menu is achieved by GO TO 1, which targets the non-existent line 1, causing the interpreter to restart the program from line 10.

Line(s)	Purpose
10–90	Initialization, color name array, menu display, and input
100	Computed GO TO dispatching to pattern blocks
1000–1550	Color bars pattern
2000–2050	Horizontal lines pattern (also called by crosshatch)
3000–3040	Vertical lines pattern (also called by crosshatch fall-through)
4000	Crosshatch — redirects to line 2000
5000–5040	Dot grid pattern
6000–6050	Border-drawing subroutine

Menu and Dispatch

Line 90 reads a keypress with INKEY$ and converts it to a number using CODE INKEY$-48, validating that r is between 1 and 5. Line 100 uses a computed GO TO expression: GO TO (r*1000), which jumps directly to the appropriate pattern block. This is a clean and idiomatic Spectrum BASIC dispatch technique.

Color Bars (Lines 1000–1550)

Eight color bars are drawn by looping c from 0 to 7, setting PAPER c, and printing spaces across a 4-column-wide, 22-row-tall region. The variable b tracks the current column offset, incrementing by 4 each iteration. After the last bar (b>28), the ink is set to white and the border subroutine is called. Color abbreviations are stored in DIM c$(8,3) and printed over each bar using INK 9 (transparent ink on Spectrum), which allows the bar’s own paper color to show through while the ink takes the current foreground.

Line and Dot Patterns

The horizontal lines pattern (line 2000) uses PLOT/DRAW to draw lines at Y positions from 3 to 173 in steps of 17 pixels. The vertical lines pattern (line 3000) similarly plots from X=0 to 255 in steps of 17. The dot grid (line 5000) uses nested loops over the same step values to plot individual pixels at each grid intersection. All three patterns use a keypress wait loop (IF INKEY$="" THEN GO TO ...) before returning to the menu.

Crosshatch Code Sharing

Option 4 (crosshatch) is handled by GO TO 2000 at line 4000. Within the horizontal lines routine at line 2035, the condition IF r=4 THEN GO TO 3010 checks whether the crosshatch option was selected, and if so, skips the keypress wait and falls into the vertical lines drawing code. This reuses both drawing routines without duplicating code, at the cost of coupling the two sections through the shared variable r.

Border Subroutine (Lines 6000–6050)

The subroutine draws a rectangular border by chaining four DRAW commands from an initial PLOT 0,0: right 255 pixels, up 175, left 255, and down 175. This traces the full perimeter of the screen area in a single connected path. It is called both during initialization and after the last color bar is rendered.

Bugs and Anomalies

• The REM at line 10 misspells “Patterns” as “Pattrens”. The menu also misspells “Horizontal” as “Horizonal” and “Monitor” as “Moniter” — these are cosmetic display errors.
• The program draws vertical lines starting from line 3000 with PLOT x,3: DRAW 0,170 using NEXT x on the same line (3020), which is valid but condensed. However, the crosshatch entry point at line 3010 skips the CLS and setup at line 3000, which is intentional since the horizontal lines are already drawn.
• INK 9 on line 1540 is a transparent ink value specific to the Spectrum attribute system, making the color name labels readable against their respective colored bars.
• Line 1035 calls GO SUB 6000 inside the NEXT c loop only when b>28 (i.e., after the eighth bar). Since b reaches 28 after the seventh iteration and 32 after the eighth, the condition triggers once, after the final bar.