Depreciation and Income Taxes

This program calculates depreciation schedules and income taxes for a capital asset, comparing two depreciation methods side by side: Straight-Line (SL) and Sum-of-Years-Digits (SOYD). The user inputs sales, cost of goods sold, operating expenses, asset cost, scrap value, and asset life; the program then iterates year by year, decrementing the remaining life counter and recomputing SOYD depreciation each pass. Income tax is computed via a subroutine at line 411 using a bracketed marginal-rate table with five bands (17%, 20%, 30%, 40%, and 46%), consistent with late-1970s/early-1980s US corporate tax schedules. Results are printed in a two-column tabular format for each year until the asset life reaches 1, at which point the program halts.

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Program Analysis

Program Structure

The program is organised into three logical phases: input collection (lines 5–48), per-year computation and output (lines 70–410), and a shared tax subroutine (lines 411–424). A single GOTO 90 at line 410 forms the main loop, re-entering at the SOYD depreciation calculation after decrementing N, so the loop body spans lines 90–410. The loop terminates via the IF N=1 THEN GOTO 430 guard at line 390, which branches to STOP.

Depreciation Calculations

Two methods are computed in parallel each year:

• Straight-Line (SL): F = INT((E-S)/N) at line 70, where E is asset cost, S scrap value, and N the original life — but note that N is decremented each iteration (line 400), so from year 2 onward the divisor shrinks and the SL figure is recalculated incorrectly. A correct implementation would preserve the original life in a separate variable.
• Sum-of-Years-Digits (SOYD): G = (N²+N)/2 at line 80 (the sum of digits 1…N), then H = INT(N/G*(E-S)) at line 90. Because N is the remaining life each pass, this is the standard SOYD formula and behaves correctly.

Tax Subroutine

The subroutine at lines 411–424 implements a five-bracket marginal tax schedule. The caller stores the taxable income in T and reads the result from T1. The brackets are:

Income range	Base tax	Marginal rate
≤ $25,000	$0	17%
$25,001–$50,000	$4,250	20%
$50,001–$75,000	$9,250	30%
$75,001–$100,000	$16,750	40%
> $100,000	$26,750	46%

This schedule closely matches US corporate income tax rates in effect circa 1978–1981. The subroutine is called twice per year (lines 112 and 115) with the SL and SOYD taxable incomes respectively, using the variable T as both input and working storage.

Key BASIC Idioms

• The ** operator (line 80) is used for exponentiation to compute N², standard on ZX81/TS1000 BASIC.
• INT() is applied throughout to truncate fractional currency values to whole dollars.
• The echo-print pattern (INPUT X followed immediately by PRINT X) is used for all inputs (lines 15–16, 25–26, etc.) to confirm entries on the display, a common ZX81 idiom since INPUT does not leave the value visible after entry.
• A GOSUB/subroutine pattern (lines 112, 115 → 411–424) avoids duplicating the tax bracket logic.

Notable Bugs and Anomalies

• SL depreciation divisor error: Line 70 uses the current (decremented) value of N rather than the original asset life. From year 2 onward, F will be larger than the correct annual SL charge, overstating depreciation and understating income.
• Uninitialised Z: The year counter Z is never explicitly set to zero; it relies on the system initialising numeric variables to 0, which ZX81 BASIC does, so this works in practice.
• Dead code at lines 126 and 140: GOTO 140 at line 126 jumps to line 140, which is the very next executed line anyway; the GOTO is redundant. This suggests the program was edited and some intervening lines were removed.
• Net income calculation uses residual T: Line 140 computes M = T - K, but at this point T holds J (the SOYD taxable income) from the second subroutine call, not I. The correct SL net income should be I - K; using T instead produces J - K, mixing SOYD income with SL tax.