Clone

CLONE is a tape utility program that loads and interfaces with a machine code block to provide three functions: block tape copying (EAR to MIC), tape header reading and display, and a “Last Resort” loader. On startup, it detects whether the host machine is a Timex model by reading five bytes from address 4413 and comparing them to the string “Timex,” then branches to alternate machine code entry points accordingly. The header reader decodes standard tape header fields from memory at address 32800 onward, displaying file type, name, length, and autostart LINE number for programs, as well as array variable names and dimensions. A calibration routine at line 8000 reads system timer bytes at addresses 23670–23671 to compute a timing adjustment, patching NOP sleds and a JP instruction into the machine code at address 57780 to compensate for tape speed variation.

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Program Analysis

Program Structure

The program is organized into several functional sections, controlled by menu-driven branching:

1. Lines 6–12: Initialization — clears memory, loads the machine code block, detects hardware type, and waits for a keypress.
2. Lines 20–90: Main menu — offers Block Copy (B), Header Reader (H), and Last Resort (L).
3. Lines 100–280: Header Reader — reads and decodes a tape header from memory at address 32800, with output routed to printer or screen.
4. Lines 2000–2070: Block Copy menu — sets tape signal polarity, launches the copy routine, or enters calibration.
5. Lines 8000–8900: Calibration — measures tape timing deviation and patches the machine code to compensate.
6. Line 9999: Save routine — saves the BASIC loader and machine code block twice each, presumably for reliability.

Hardware Detection

Lines 7–9 perform a Timex model detection. After an initial RANDOMIZE USR 57472 call, five bytes are read from address 4413 into the string s$. If s$="Timex", two POKEs redirect subsequent machine code calls to alternate entry points (57532,192 and 57533,26 instead of 57532,125 and 57533,27). This technique reads from the ROM identification area to distinguish Timex hardware, and the result is reused throughout the program at lines 150–151.

Machine Code Interface

The program relies heavily on a machine code block loaded at address 57472 (with a corresponding BASIC save of "cloneO" CODE 57472,8064 at line 9999). Multiple entry points into this block are used:

Address	Purpose
57472	Initial setup / hardware probe
57491	Header read (non-Timex)
57550	Header read (Timex)
57589	Calibration timing measurement
57735	Block copy (EAR to MIC)
58080	Post-load initialization
65004	Block load launcher

Header Decoding

Lines 160–265 decode the standard 17-byte tape header, which the machine code has deposited at address 32800. The byte at 32800 indicates file type (0=Program, 1=Number array, 2=Character array, 3=Bytes). The filename occupies bytes 32801–32810; a trailing-space trimmer at lines 210–240 scans backward to find the last non-space character (mark), then prints only up to that position.

Additional fields are decoded at fixed offsets:

• PEEK 32811 + 256*PEEK 32812 — data length
• PEEK 32813 + 256*PEEK 32814 — parameter 1 (LINE autostart or load address)
• PEEK 32815 + 256*PEEK 32816 — parameter 2 (program area offset for PROGRAM type)

For number arrays, CHR$ (PEEK 32814-64) recovers the variable letter. For character arrays, CHR$ (PEEK 32814-128) is used instead, reflecting the different encoding of array variable names in the header.

Line 250 suppresses printing the LINE number when it is 9999 or above, which is the conventional sentinel value indicating no autostart line is set.

Tape Speed Calibration

The calibration section (lines 8000–8120) reads two system timer bytes from addresses 23670 and 23671 after RANDOMIZE USR 57589 completes. The difference dif is used in the formula:

makeup = INT((29*dif/2 - 49) / 4)

This yields a count of NOP bytes to insert as a timing pad. Lines 8050–8120 fill addresses from 57780 onward with 0 (NOP, opcode 0x00), then write a JP nn instruction (opcode 0xC3 = 195, followed by address bytes 151 and 225, i.e., jump to 57751) at the end of the NOP sled. The value of makeup is clamped between 0 and 290 to prevent overwriting unintended memory.

Polarity Control

Lines 2020–2030 allow toggling the tape signal polarity for the copy function by POKEing pairs of bytes at 57749 and 57750. Normal polarity uses values 95 and 31; inverted polarity reverses them (31 and 95). Line 2010 displays the current setting by using the AND conditional string trick: "Normal " AND PEEK VAL "57749"=VAL "95" evaluates to the string if the condition is true, or an empty string otherwise.

Key BASIC Idioms

• VAL "number" in GO TO, AT, and POKE arguments at lines 2000–2010 is a memory-saving technique, storing numeric literals as shorter string tokens.
• The keypress wait loop pattern at lines 11–12 (flush any held key, then wait for a new press) is standard practice.
• OPEN #2,n$ at line 140 routes subsequent PRINT output to either the printer ("p") or screen ("s") depending on the user’s earlier choice.
• Line 270 uses POKE 23692,5 after each header read to prevent the “scroll?” prompt from interrupting continuous header display.

Bugs and Anomalies

• Lines 30–90 form a polling loop, but line 40 handles the “B” key inline without a GO TO skip — after RANDOMIZE USR 65004 returns (if it does), execution falls through to line 50 and then 90, looping back. This is likely intentional, as the machine code at 65004 may not return.
• Line 150’s POKE 23692,5 resets the line counter before each header read, but the loop at GO TO 150 from line 280 will continuously read headers with no exit condition; the only way out is to stop the tape and let a read fail or error out.
• The save line (9999) saves the BASIC program under the name "CLONE" twice and the code block under "cloneO" twice, which provides redundancy on tape but could confuse loaders expecting a single copy.