Job Jar

This program implements a “Job Jar” task management system, allowing the user to initialise a job list, add job descriptions with time estimates, mark jobs as done (removing them from the list with an array shift), pick a random job matching available time, list all jobs, and save the program to tape. Job entries are stored in a dimensioned string array B$ with 28 characters per record: 24 characters for the description and 3 characters (positions 26–28) for a numeric time estimate. The PICK function (line 4000) implements a rudimentary time-matching and randomised selection algorithm that decrements the requested time by one each pass until a matching job is found, using RND to introduce non-determinism. The save routine at line 9550 renders a pixel-accurate bitmap of the text “JOB” by reading character ROM data via PEEK from address 7680, printing inverse or normal space characters to reconstruct each glyph on screen.

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Program Analysis

Program Structure

The program is organised as a menu-driven application with a central dispatcher at line 490–600 and several functional modules at higher line numbers. Shared subroutines at lines 10–70 handle common prompts (Y/N confirmation, “More?”, “Press C”, and printer copy). The main modules are:

1. Lines 490–600: Main menu and dispatcher
2. Lines 1000–1050: Initialise — dimension the job array
3. Lines 2000–2170: Add a job entry
4. Lines 3000–3160: Mark a job done and remove it
5. Lines 4000–4260: Pick a random job by available time
6. Lines 5000–5050: List all jobs
7. Lines 9550–9680: Save program and display logo splash screen

Data Storage

Jobs are stored in a two-dimensional string array B$ dimensioned at line 1030 as DIM B$(N1,28). Each record uses a fixed 28-character layout:

Positions	Content
1–24	Job description (free text)
25	Unused / padding
26–28	Time estimate (numeric string, right-padded with spaces)

A scratch buffer X$ of 28 characters is dimensioned at line 2020 for each add operation, then copied wholesale into B$(N) at line 2140.

Input Validation

Both the ADD and PICK routines validate their numeric inputs by iterating character-by-character over the time-estimate field. The condition at lines 2070 and 4050, IF X$(K)<"A" AND X$(K)>".", attempts to accept only digits (which lie between "." and "A" in the ZX81 character set). Spaces are accepted as padding after the first character (lines 2064, 4044). This validation is applied in a FOR loop but uses GOTO to skip the first character check, which skips validation for position 26 entirely — a minor anomaly that could allow a non-digit leading character.

Job Deletion by Array Compaction

When a job is marked done (line 3000 module), the array is compacted by shifting all subsequent entries down one position (lines 3070–3090), then decrementing the job counter N at line 3100. This is a classic in-place list deletion without pointer arithmetic, achieved purely in BASIC.

Random Job Picker Algorithm

The PICK routine (lines 4085–4260) is a two-phase search. It scans all N jobs for one whose time estimate string matches X$(1 TO 3). If none is found and the search has not already been expanded (X=1 at line 4130), it decrements the requested time by 1 and retries — effectively widening the search downward. Once a matching job is found (line 4110), it is selected only with probability 1/3 using IF INT(RND*3+1)<>1 at line 4200, providing randomisation across multiple matches. The variable X is set to 1 at line 4090 on the assumption that the loop sets it, but there is a bug: X is only reset to 0 at line 4090 at the top of each retry pass, never explicitly set to 1 in the no-match path; line 4130 checks X=1 which will never be true, so the decrement loop runs indefinitely if no match is ever found.

Additionally, line 4160 contains IF T=0 THEN LET T1=T1+1, which increments T1 when the counter reaches zero, but this value is never used thereafter — this appears to be dead or vestigial code.

Character ROM Logo Renderer

The save splash screen (lines 9565–9645) renders the letters “JOB” pixel-by-pixel by reading the character ROM. For each letter in A$="JOB", it retrieves the ASCII code with CODE A$(NN) and reads 8 bytes from address 7680 + C*8 + H via PEEK. Each byte is decomposed bit-by-bit by repeatedly testing against a bitmask V (starting at 128 and halved each iteration). Set bits are printed as inverse space characters ("% ") and clear bits as a block graphic ("\@@" = two @ characters rendered as a light block). The three letters are spaced 7 columns apart starting at column 5, and rendered starting at row 3, producing a large pixelated title.

Subroutine Conventions and GOSUB Usage

Three shared subroutines are invoked repeatedly:

• GOSUB 10 — “CHANGE Y OR N?” prompt; sets flag F=1 for Y, F=0 for N
• GOSUB 30 — “MORE Y OR N?” prompt; falls through to line 14 of subroutine 10
• GOSUB 40 — “PRESS C TO CONTINUE” prompt
• GOSUB 60 — “COPY Y OR N?” prompt with optional COPY to printer

Subroutine 30 at line 34 jumps directly into the body of subroutine 10 at line 14, sharing its Y/N polling loop and RETURN — a deliberate code-reuse technique via fall-through.

Key BASIC Idioms

• Inverse-video text in messages (e.g. "%B%A%D% %E%S%T%I%M%A%T%E") uses the ZX81 inverse character escape to highlight error notices.
• RAND 0 at line 490 seeds the random number generator to a fixed value, ensuring reproducible picks each session start.
• SLOW / FAST mode switching is used around the display-intensive logo renderer and the main menu to balance display quality versus computation speed.
• The copy subroutine at line 62 uses a double-poll pattern (IF INKEY$<>"" THEN GOTO 62 followed by IF INKEY$="" THEN GOTO 64) to wait for a key release then a new keypress, debouncing the input.