5 Interesting Routines

This program is a menu-driven collection of six utility routines for the TS2068, selectable from a central dispatcher at line 2500. Two “reader” routines (lines 10 and 100) walk the BASIC program area by peeking system variables at addresses 23635/23636 (PROG) and 23627/23628 (VARS), displaying each byte’s address, raw value, and printable character; Reader 2 additionally decodes line headers, line lengths, and embedded five-byte floating-point number tokens using a DATA-driven label lookup. Lines 500 and 1000 implement two custom INPUT routines—one character-at-a-time with cursor positioning and one that fills a dimensioned string array—both using the PAUSE 0/INKEY$ keypress idiom with backspace support. Line 1500 loads and executes 25 bytes of Z80 machine code (checksum-verified at 2651) into RAM at address 40000 to print all 28 BASIC error messages, and line 2000 performs a 16K ROM integrity check using a byte sum (expected 1752253), a zero-byte count (expected 609), and a floating-point arithmetic error accumulation test.

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Program Analysis

Program Structure

The program is organized as six self-contained routines sharing a single file, dispatched by a menu at line 2500. Line 5 jumps directly to the menu on first run. Each routine occupies a distinct line-number band, separated by REM banners, and each returns to the menu via GO TO 2500 or RUN/PAUSE 200: RUN.

Lines	Routine	Menu Option
10–70	Reader 1 — raw byte dump of BASIC program area	1
100–250	Reader 2 — structured BASIC line decoder	2
500–630	Input 1 — single-string character editor	3
1000–1110	Input 2 — array string editor	4
1500–1630	Error Messages — machine code loader	5
2000–2310	ROM Check — 16K ROM integrity tests	6
2500–2590	Menu dispatcher	—

Note a minor bug in the menu: option 6 is listed in the PRINT at line 2520, but the INPUT validation at line 2530 only accepts values 1–5 (a>5 rejects 6), making ROM Check unreachable through the menu. Line 2590 (RUN 2000) is therefore dead code under normal menu navigation.

Reader 1 (Lines 10–70)

This routine uses system variables PROG (address 23635/23636) and VARS (23627/23628) to establish the boundaries of the BASIC program area. It iterates byte-by-byte from PROG to VARS, printing each address and raw byte value. Characters with codes 16–23 (Spectrum/TS2068 control codes for INK, PAPER, FLASH, etc.) are labeled (unprintable); all others are rendered with CHR$. After reaching VARS, it pauses 200 frames and restarts.

Reader 2 (Lines 100–250)

Reader 2 provides a more structured dump, decoding the BASIC line format: two-byte line number (big-endian), two-byte line length (little-endian), then token bytes. The subroutine at line 240 prints the current address and its byte value in columnar form. Special handling is provided for token 14 (the embedded five-byte floating-point literal): it triggers a RESTORE and reads five labels from the DATA statement at line 250 ("FIVE","BYTE","EQUIV.","OF","NUMBER"), printing one label per byte. End-of-program is detected when p=v (VARS boundary).

Input 1 (Lines 500–630)

This routine implements a character-by-character string input with on-screen cursor, using PAUSE 0/INKEY$ at line 560. A flashing ? serves as the cursor. Backspace (CHR$ 12) deletes the last character by slicing the string with t$(TO LEN t$-1) and overwriting the screen position with a space. The screen-wrapping logic at line 540 is notable: it tests y>31 OR y=-1 and adjusts row (x) and column (y) accordingly, supporting multi-line input.

Input 2 (Lines 1000–1110)

This routine fills a two-dimensional string array t$(n,l) (10 rows × 15 columns) one character at a time. Each row is pre-filled with hyphens (CHR$ 45) as a visual field indicator. Backspace reduces k and truncates the row string. Unlike Input 1, wrapping between rows is handled by the FOR j loop structure rather than explicit coordinate arithmetic.

Machine Code Error Message Printer (Lines 1500–1630)

Line 1510 sets the load address to l=40000. A FOR loop POKEs 25 bytes of Z80 machine code into RAM, accumulating a checksum. Line 1560 verifies the checksum equals 2651 before executing with RANDOMIZE USR l. The DATA bytes decode as:

1. 06 1C — LD B,28 (loop counter for 28 error messages)
2. C5 — PUSH BC
3. CD A9 08 — CALL 0x08A9 (ROM routine)
4. C1 — POP BC
5. 78 — LD A,B
6. 11 65 0F — LD DE,0x0F65
7. C5 — PUSH BC
8. CD 3F 07 — CALL 0x073F (print routine)
9. 06 32 — LD B,50
10. 76 — HALT (delay loop)
11. 10 FD — DJNZ (back to HALT)
12. C1 — POP BC
13. 10 EB — DJNZ (outer loop)
14. CF — RST 8 (error restart)
15. 1A — error code 26

Lines 1580–1610 contain developer notes and commented-out hex strings for an alternative machine code version, and line 1620 contains a MOVE command and OUT 244 calls indicating TS2068-specific cartridge/bank switching that appears to be unfinished development code.

ROM Check (Lines 2000–2310)

Three independent checks are performed against the 16KB ROM (addresses 0–16383):

• Byte sum check: accumulates PEEK i for all 16384 bytes; expected value 1752253.
• Zero-byte count: counts bytes equal to zero; expected value 609.
• Floating-point arithmetic error test: loops from 1000 to 1100 computing i*0.5 - i/2, i*0.1 - i/10, and i - SQR(i*i), which should each yield zero for a correct ROM but accumulate rounding errors. The combined average is compared against a hard-coded expected error of 1.3391177E-7.

The arithmetic error test is particularly clever: it exploits the fact that floating-point rounding behavior is deterministic for a correctly burned ROM, making accumulated error a fingerprint of ROM fidelity rather than just content. The comment at line 2300 notes that the additional 8K ROM for cartridge and bank-switching functionality is not tested.

Notable BASIC Idioms

• PAUSE 0: LET q$=INKEY$ — efficient single-keypress wait at lines 560, 1040.
• 256*PEEK addr + PEEK (addr+1) vs. PEEK addr + 256*PEEK (addr+1) — both big-endian and little-endian 16-bit reads are used appropriately for line numbers and line lengths respectively.
• RESTORE without a line number at line 180 resets the DATA pointer to the first DATA statement, which here is line 250 — a side effect that works correctly only because no other DATA is read before this point in routine 2’s execution.
• Checksum verification before RANDOMIZE USR is a sound defensive practice for machine code loaders.