Binary

This program demonstrates binary number representation by displaying an 8-bit “data bus” on screen, where each bit position is shown as a coloured column that lights green (PAPER 4) or yellow (PAPER 6) depending on whether that bit is set. The bit-extraction routine at line 310–370 uses successive halving of a power-of-2 variable (starting at 128) and subtraction to determine each bit’s value without using bitwise operators, which are unavailable in Sinclair BASIC. A demonstration mode (subroutine 500) counts automatically from 1 to 255, while a calculate mode (subroutine 600) accepts direct decimal input and validates the range 0–255. The decimal value is displayed with a zero-padded two-digit format constructed by prepending “00” to the string and taking the last three characters, a common Sinclair BASIC padding idiom.

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Program Structure

The program is organised into clearly separated subroutines, each prefaced with REM comment blocks:

Lines	Role
10–11	Title REMs
110–160	Main loop: initialisation and menu dispatch
300–390	Core display routine: render binary bits and decimal value
400–460	Screen setup: borders, column dividers, labels
500–550	Demonstration mode: auto-increment 1–255
600–640	Calculate mode: validated decimal input

Execution begins at line 110, which calls the screen-setup subroutine (400) and then the display routine (300) with number=0 before entering the menu loop.

Bit-Extraction Algorithm (Lines 310–370)

Because Sinclair BASIC provides no bitwise AND or shift operators, the subroutine at line 310 implements manual bit decomposition using subtraction:

1. Initialise power=128 and remain=number.
2. For each bit position x (0 to 7): subtract power from remain.
3. If remain goes negative, restore it by adding power back and set result=0; otherwise result=1.
4. Halve power and advance to the next bit.

This is the standard “successive subtraction” equivalent of a right-shift and mask in assembly language.

Screen Layout and Graphics

The setup routine at line 420 uses a combined FOR loop to draw all eight bit columns in one pass. For each column x, it prints the bit label (d7 down to d0) and the corresponding power-of-two value, then uses PLOT/DRAW to draw vertical separator lines at pixel positions 32*x+7 and 32*x+24, spanning from y=96 to y=151 (55 pixels high). A horizontal rule is drawn at y=174 across the full data bus area, and another at y=64 beneath the binary digit row.

Each bit column occupies four character cells of width (z=x*4+1), giving eight columns across the 32-column screen with one column of margin on each side.

Colour Coding of Bits

Line 350 computes color=4-2*result, yielding PAPER 4 (green) when result=1 and PAPER 2 (red) when result=0. Wait — PAPER 4 is green and PAPER 2 is red in the standard Spectrum palette. Each lit column is printed over rows 3–9 (seven character rows), making the bit columns visually prominent.

Zero-Padded Decimal Display

Line 380 constructs a zero-padded three-character decimal string using the idiom:

LET a$="00"+STR$ number: LET a$=a$(LEN a$-2 TO LEN a$)

Prepending “00” and then slicing the last three characters produces “000”–”255″. This avoids conditional branching or IF chains to handle different digit counts.

Demonstration Mode (Lines 500–550)

The demo loop iterates a from 1 to 255, calling the display subroutine each time. A freeze feature is implemented at line 540: if any key other than M/m is held, INKEY$ is non-empty and the program busy-waits in a tight GO TO 540 loop until the key is released. Pressing M or m exits the subroutine. Note that the loop does not reset to 0 after reaching 255; it simply returns, leaving the display showing 255.

Input Validation (Lines 620–630)

User input is taken with INPUT and then truncated with INT to discard any fractional part. The condition number>255 OR number<0 re-prompts for out-of-range values. However, no protection is given against non-numeric input, which would cause a BASIC error rather than a graceful re-prompt.

BEEP Usage

Line 390 ends the display subroutine with BEEP .5,number/4, producing a half-second tone whose pitch scales with the current value. For number=0 this produces a 0-semitone (middle C) tone; for 255 it produces a tone 63.75 semitones above middle C. This provides audio feedback in both demo and calculate modes.