Financial

This listing contains four separate financial calculation programs: a loan amortization scheduler, a Rule of 78s interest calculator, a compound interest plotter, and a straight-line/sum-of-years depreciation table generator. The amortization program uses a shared subroutine at line 10 to normalize floating-point numbers to two decimal places by examining the string representation produced by STR$, compensating for the lack of built-in currency formatting. The compound interest program uses PLOT to draw a graphical chart of investment growth, dynamically scaling the Y-axis and adjusting the X-axis tick labels based on whether the term is ≤12, 13–25, or >25 years. A hardware status check via PEEK 16442 in the amortization loop triggers a COPY/CLS subroutine at line 500 when display memory is nearly full, preventing scroll overflow during long payment schedules.

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Program Analysis

This file contains four independent BASIC programs saved sequentially, each self-contained with its own SAVE and re-entry GOTO at the end. They share a common origin (UAS, Haddonfield NJ) and several coding idioms. Each program is analysed in turn.

Program 1 – Amortization Scheduler (lines 10–2010)

Calculates and prints a loan amortization table, solving either for the monthly payment (given term) or for the term (given payment). The core payment formula uses the standard annuity present-value equation at line 150, and the term-solving path at line 440 inverts it using LN.

Structure

1. Lines 10–24: Subroutine — normalise a float X1 to two decimal places and return its string length in Z.
2. Lines 25–95: Title banner, input collection (loan amount, rate, periods/year, mode choice).
3. Lines 100–175: Initialise balance, compute periodic interest rate I, compute payment P (or skip if solving for term).
4. Lines 180–310: Input print-depth D, main amortization loop printing each period’s payment, interest, principal, and balance.
5. Lines 320–341: Final period handling — last payment clears the residual balance exactly.
6. Lines 350–379: Summary, re-run prompt, NEW on exit.
7. Lines 400–450: Alternate entry for “solve for term” mode.
8. Lines 500–580: Subroutine — prompts for COPY (print), then CLS; uses SLOW/FAST to ensure a clean print.

Notable Techniques

• Decimal normalisation subroutine (lines 10–24): Because STR$ may produce "5", ".5", "5.5", or "5.50", the subroutine examines the string character by character to determine how many characters short of two decimal places the result is, then adds the deficit to Z so callers can back-compute correct TAB positions. It does not pad the string itself — it only adjusts the column offset used in the subsequent PRINT TAB.
• Display-full guard (line 265): IF PEEK 16442<4 THEN GOSUB 500 — location 16442 (DFILE pointer low byte) is checked to detect that the display file is close to overflowing the 24-line screen. When true, a COPY/CLS cycle is triggered.
• Clamping D to N (line 200): LET D=D+(N-D)*(D>N) uses a Boolean-as-integer idiom (true=1, false=0) to silently cap the print-depth at the total payment count without an IF statement.
• Loop endpoint adjustment (line 260): FOR J=1 TO D-(N=D) — again uses Boolean arithmetic; when printing all periods, the loop runs to D-1 and the final period is handled separately (lines 330–341) so the last payment can be adjusted to clear any floating-point residual in the balance.
• INKEY$ debounce (lines 520–540): Three-stage loop — flush any held key, wait for a keypress, validate it is Y or N — preventing accidental COPY triggering.

Program 2 – Rule of 78s Interest Allocator (lines 10–510)

Computes the cumulative interest rebate schedule under the Rule of 78s (sum-of-digits) method. For a loan with N payments and total interest I, it iterates from payment N down to 1, accumulating digit sums to derive the interest earned in each period and the interest remaining.

Structure

1. Lines 10–90: Subroutine — append "0" if string ends one digit after the decimal point, or append ".00" if no decimal present. Updates H to the new length.
2. Lines 91–94: Title, pause, clear.
3. Lines 100–165: Input interest total and number of payments.
4. Lines 170–260: Compute Z=N*(N+1)/2, then loop printing cumulative interest paid and interest remaining.

Notable Techniques

• This program’s decimal subroutine (lines 10–90) modifies A$ directly by appending characters, unlike Program 1’s subroutine which only adjusts a column counter.
• The loop runs FOR K=N TO 1 STEP -1 and accumulates M=M+K, so M/Z gives the fraction of total interest attributed to periods 1 through N-K+1 — a clean implementation of the sum-of-digits method.
• Column alignment uses TAB 13-H and TAB 23-H where H is the post-normalisation string length, right-aligning the two money columns.

Program 3 – Compound Interest Graph (lines 10–3010)

Accepts investment amount, annual rate, periods per year, and term in years, then plots a graphical growth curve using PLOT against drawn axes, with dynamic Y-axis scaling and X-axis labels adapted to the term length.

Structure

1. Lines 10–90: Subroutine (same decimal normalisation as Program 1’s version).
2. Lines 450–480: Title banner and CLS.
3. Lines 500–630: Input collection; compute effective annual growth factor B=(I/100/P+1)**P.
4. Lines 640–710: Compute final value A1; if too small for the plot grid, multiply by 10 and note the scaling.
5. Lines 720–860: Draw Y-axis (vertical PLOT), X-axis (horizontal PLOT), tick marks, and Y-axis labels.
6. Lines 870–940 / 1500–1610 / 2000–2070: Three branches select X-axis tick spacing and labels based on term (≤12, 13–25, >25 years).
7. Lines 20–190: Subroutine entry point — plot the growth curve, print final yield, prompt to repeat.

Notable Techniques

• Ceiling division (line 700): LET X=(INT (A1/32))+((A1/32)>INT (A1/32)) computes ⌈A1/32⌉ using Boolean arithmetic, ensuring the Y-axis scale unit always covers the full range.
• Incremental plotting (lines 60–130): Rather than computing each Y value from scratch, the loop tracks the previous bar height in B1 and distributes the height gain evenly across the H pixels in each interval using integer division, producing a smooth stepped curve.
• Inverse-video title (line 150): "%Y%I%E%L%D%S" prints “YIELDS” in inverse video characters, immediately followed by the numeric yield ratio.
• The REM at line 10 contains a quoted "X" — this is a common ZX81/TS1000 technique to store a single byte (here the character X) inside a REM for later PEEK/POKE use, though it is not used programmatically here.
• CHR$ 7 at lines 910/1520/2040 prints the ↑ (up-arrow / exponentiation) character, used decoratively as a tick mark on the X-axis.

Program 4 – Depreciation Table (lines 100–710)

Prints a side-by-side comparison of straight-line depreciation and sum-of-years-digits depreciation for a fixed asset, showing annual depreciation charge and book value for each year.

Structure

1. Lines 100–130: Title, pause, CLS.
2. Lines 200–320: Input cost C, scrap value SV, years N; precompute straight-line charge DC and sum-of-digits denominator S.
3. Lines 490–510: Column headers.
4. Lines 520–670: Main loop — each iteration prints year number, straight-line depreciation and running book value, then sum-of-years depreciation (T/S * (C-SV)) and its running book value.
5. Line 680: STOP.

Notable Techniques

• Column alignment is handled throughout with the pattern TAB (N-LEN A$) after converting the value to a string, right-aligning numbers in fixed-width columns without any padding subroutine.
• The sum-of-years fraction at line 610 is T/S where T=N+1-K counts down the remaining useful years — a textbook implementation of the SYD method.
• Unlike the other programs, this one ends with STOP at line 680, with the SAVE/GOTO pair unreachably at lines 700–710, suggesting the STOP was added during development and the save lines were not updated.

Bugs and Anomalies

Program	Location	Issue
Amortization	Line 540	IF INKEY$<>"Y" AND INKEY$<>"N" samples INKEY$ twice in one statement; since INKEY$ is re-evaluated each reference, the two reads may return different values if keys are transitioning.
Amortization	Lines 145/180	When Z$="B", the program jumps from line 145 to 180, skipping line 150 where P would be computed — but line 400 has already set P from input, so this is intentional.
Compound	Line 55	IF N>T THEN GOTO 150 exits the outer loop early when the number of plotted periods exceeds the term, but N is also used as the loop counter for the compound periods — reuse of N across different contexts could cause confusion if the program is modified.
Depreciation	Line 680	STOP makes lines 700–710 (SAVE and re-entry GOTO) unreachable during normal execution.