Savings Plan

This program calculates and displays a month-by-month savings plan based on user-supplied initial balance, monthly deposit amount, annual interest rate, and number of months to simulate. Each iteration calls a subroutine at line 520 that adds the monthly deposit, computes interest by dividing the annual rate by 1200, and rounds both the interest and running balance to two decimal places using the INT(x*100+0.5)/100 idiom. A cumulative interest accumulator is maintained across all months, with a final summary printed after the loop. The program offers a restart option that returns to line 100, resetting the display but re-entering the same subroutine without reinitialising all variables.

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Program Analysis

Program Structure

The program is organised into three logical sections:

1. Introduction and input (lines 100–330): prints a description, then collects the initial amount (J), monthly savings amount (P), yearly interest rate (I), and number of months (M).
2. Display loop (lines 340–460): clears the screen, prints headers, and iterates from 1 to M, calling subroutine 520 each pass and printing per-month data.
3. Subroutine (lines 520–600): performs the core financial calculation — adding the deposit, computing and rounding interest, updating the balance, and accumulating cumulative interest.

Variable Usage

Variable	Purpose
J	Initial amount entered by the user; preserved for display
B	Running balance, initialised to J
P	Monthly savings amount (deposit)
K	Copy of P, used only in the header display
I	Annual interest rate as a percentage
MI	Monthly interest rate as a decimal ((I/12)/100)
IN	Interest earned in the current month
CI	Cumulative interest across all months
M	Number of months to display
A	Loop counter (reused as restart yes/no flag at line 490)
T	Inner delay loop counter

Rounding Technique

Both the monthly interest and the running balance are rounded to two decimal places using the classic idiom INT(x * 100 + 0.5) / 100 (lines 550, 570–580). The half-up rounding on the balance at line 570 uses INT(B*100+0.5) and then divides by 100 on the next line, splitting the operation across two statements — likely for clarity or to stay within a line-length limit.

Delay Loop

Lines 420–430 implement a busy-wait delay of 400 iterations after printing each month’s row (FOR T=1 TO 400: NEXT T). This was a common technique to slow scrolling output to a readable pace on machines without a built-in PAUSE command or where PAUSE required a keypress.

Notable Idioms and Techniques

• The PRINT J / PRINT P / PRINT I / PRINT M lines immediately after each INPUT echo the entered value back to the screen, which was necessary on ZX81/TS1000 because INPUT does not permanently display the typed value after execution.
• Line 350 uses PRINT "MONTHLY SAVINGS PLAN" — the is a zmakebas escape representing character code 72 (the letter ‘H’), which appears to be a stray character or cursor-positioning artefact rather than intentional formatting.
• Variable K is assigned P at line 250 and used only in the header at line 370 ("MONTHLY SAVINGS AMT=$ ";K), while P itself is used in the calculation subroutine. Since P is never modified, K is redundant.

Bugs and Anomalies

• Typo in header: Line 360 prints INTEREST R8TE — the digit 8 appears in place of the letter A, a likely keyboard entry error.
• Typo in prompt: Line 310 reads "ENTER THE NR OF MONTHS TK BE DISPLAYED" — TK should be TO.
• Restart does not reset state: When the user enters 1 at line 490, the program jumps to line 100, which re-inputs all values but does not reinitialise CI (cumulative interest) or B (balance). CI is only set to 0 at line 131, which is bypassed on restart. This means a second run will carry over the cumulative interest and balance from the first run, producing incorrect results.
• Variable A reuse: A serves as both the FOR loop counter (lines 390–440) and the yes/no restart input (line 490). After the loop completes, A holds the value M; overwriting it with the user’s input at line 490 is harmless but potentially confusing.

SAVE Line

Line 700 contains SAVE "1002%9" where %9 is an inverse-video digit 9, serving as an auto-run flag embedded in the filename. Line 710 (RUN) would execute automatically after loading.