Business #4

This listing contains two independent business finance programs. The first performs break-even analysis, prompting the user for fixed costs, variable parts cost per unit, selling price, starting and finishing stock quantities, and an increment step, then printing a columnar cost/revenue/profit-loss schedule with a clearly marked break-even row; it supports optional hard-copy output via LPRINT. The second program is a job quote estimator that collects materials, shop labour, and on-site labour costs (each optionally itemised line-by-line), then computes overhead and profit as user-supplied percentages and prints a formatted multi-column estimate summary, with a COPY command triggered by pressing Z to produce a printer dump. Both programs use SCROLL extensively to manage the ZX81’s 24-row display in SLOW mode, switch to FAST mode for computation-heavy sections, and round currency values using the INT(100*x+0.05)/100 idiom for two-decimal-place accuracy. The itemised entry subroutine at line 1200 is shared between the materials, shop labour, and on-site labour collection paths in the second program.

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Program Analysis

The listing contains two completely independent programs separated by their own SAVE and RUN tail lines. The first (lines 10–9999) is a break-even analysis tool. The second (lines 10–9999, restarting) is a job quotation estimator. They are analysed separately below.

Program 1 – Break-Even Analysis: Structure

The program is entirely linear with no subroutines. It collects six inputs (fixed costs F, variable cost per unit V, selling price P, start quantity B, finish quantity C, and increment S), then iterates from B to C in steps of S, printing a cost/revenue table. At line 460–480 the break-even quantity D, break-even revenue G, and break-even cost H are calculated before the loop begins.

1. Input phase (lines 70–410): six prompted INPUT statements, each followed by confirmation PRINT and SCROLL pairs.
2. Hard-copy option (lines 432–457): asks for Y$; any line thereafter guarded by IF Y$(1)="Y" duplicates output to the printer via LPRINT.
3. Computation and output loop (lines 530–1030): FOR N=B TO C STEP S; at line 580 a test IF N<D branches to 1000 (skipping the break-even banner), otherwise prints the separator, break-even row, and resets D=99999999 so the banner fires only once.
4. Recycle (lines 2000–2060): waits for any keypress then RUNs from the top.

Program 1 – Notable Techniques and Idioms

• FAST/SLOW switching: FAST at line 420 speeds up the CLS and setup; SLOW at line 510 restores display before the output loop so results are visible as they scroll.
• One-shot break-even banner: after printing the break-even row (lines 590–640), D is set to 99999999 (line 650) so the condition at line 580 never triggers again within the same run.
• Zero-start guard: line 520 prevents a division-by-zero in U=E/N when B=0 by advancing B to S.
• Unit-cost rounding: line 565 applies INT(10*U+.5)/10 — rounding to one decimal place.
• Busy-wait keypress: lines 2030–2040 use the INKEY$ polling loop, a standard ZX81 idiom.
• LPRINT mirroring: every significant PRINT in the output phase has a corresponding IF Y$(1)="Y" THEN LPRINT guard, doubling the line count but keeping hard-copy output structurally identical to screen output.

Program 1 – Bugs and Anomalies

• At line 9998 the filename contains an inverse N mid-word, which is the auto-run marker embedded in the filename string.
• The LPRINT column footer at line 1060 contains leading spaces rather than using TAB, so alignment may drift slightly depending on printer character width.
• If P=V (selling price equals variable cost), line 460 will cause a division-by-zero error as there is no guard.

Program 2 – Job Quote Estimator: Structure

This program uses a counter D (line 20, incremented at line 205) to number successive estimates within a session. Three subroutine groups handle data collection and output:

Line range	Purpose
1000–1190	Material cost collection: dispatch to itemised (1200) or total-entry path
1200–1670	Generic line-item entry loop (shared by materials, shop labour, on-site labour)
2000–2550	Labour cost collection: shop labour then on-site labour, each optionally itemised
3000–3290	Quote summary display and optional COPY dump

Program 2 – The Shared Itemisation Subroutine (line 1200)

Subroutine 1200 is called three times: for materials (line 1150), shop labour (line 2115), and on-site labour (line 2340). It accumulates a running total in C, collecting item name B$, unit cost A, and quantity G in a loop. An empty B$ input (line 1255) terminates entry and returns the total in C. The caller then copies C into the appropriate variable (M, L, or H).

Program 2 – Financial Calculations

All monetary arithmetic is done in floating point and then rounded. The main formula chain (lines 550–640):

• B = Q*M — total material cost (units × per-unit material)
• K = Q*L — total shop labour
• J = H*Q — total on-site labour
• V = O*(L+H) — overhead per unit (percentage of direct labour only)
• W = V*Q — total overhead
• R = P*(B+K+J+W) — total profit (percentage of total job cost excluding profit itself)
• S = B+K+J+R+W — total quote
• T = S/Q, X = R/Q — per-unit equivalents

Note that overhead is calculated on direct labour only (L+H), not on materials, which is a deliberate business convention reflected in line 590.

Program 2 – Currency Rounding Idiom

Throughout the summary subroutine (lines 3102–3256), values are rounded to two decimal places using INT(100*x+.05)/100. The bias of 0.05 (rather than the standard 0.5 used in program 1’s line 565) is slightly too small — correct two-decimal rounding requires +0.005 — so values near half-penny boundaries may round down incorrectly. The same pattern appears in the itemisation loop at line 1455.

Program 2 – COPY Dump Mechanism

The summary subroutine ends with a PAUSE 40000 (line 3270) giving the user approximately 40 seconds to press Z. Line 3280 tests INKEY$="Z" immediately after the pause expires or the user presses a key; if Z was the key that broke the pause, COPY is executed to dump the screen to a printer. This is a common ZX81 screen-print idiom since COPY sends the full display bitmap to the printer port.

Program 2 – Anomalies

• Line 2130 uses GOTO 2240 rather than RETURN after setting L=C, skipping the corresponding RETURN at line 2230 (which does not exist). This means execution falls through to line 2240 correctly but is structurally fragile.
• “INSTALATIONS” is misspelled at line 3080 (missing one L), as it is at line 300 in the input phase.
• Variable C is overloaded: it holds the overhead percentage input at line 410 and is also the running item total inside subroutine 1200. The overhead percentage is safely converted to O=C/100 at line 430 before any subroutine call, so there is no collision in practice, but the summary at line 3200 prints C as the overhead percentage label — which by that point still holds the last value from the itemisation loop, not the original percentage. This is a latent display bug.
• The GOTO 100 at line 690 re-enters the main loop but does not reinitialise D, so the estimate counter correctly increments across quotes in a session.