Parallel 1000

Parallel 1000 is a ZX81/TS1000 printer driver installation utility that configures and relocates a Z80 machine code parallel printer driver to one of four memory locations: above RAMTOP, below system variables (address 8192), at the line 1 REM statement, or at a user-specified address. The program validates the relocation address against key system boundaries, checking that it falls between 8192 and 32767 and avoids the system variable area (15784–17325). It calculates high and low bytes of the target address using integer division and POKEs them into memory at addresses 17127 and 17128, then activates the driver via USR calls to fixed offsets (17161 or 17157). Entry points for LCOPY, LLIST, CLRBUF, and LPRINT functions are reported as USR expressions relative to the BASE address, with offsets of 167, 325, 560, and 171 respectively. The REM statements at lines 1 and 3 contain the actual Z80 machine code payload embedded in the BASIC program.

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Program Analysis

Program Structure

The program is organized into several functional blocks:

1. Lines 1 and 3 (REM blocks) — contain the embedded Z80 machine code for the parallel printer driver, stored as raw bytes within REM statement payloads.
2. Lines 1000–1110 — user interface for linefeed control code configuration and relocation mode selection.
3. Lines 1210–1280 — custom address entry and validation loop.
4. Lines 1300–1450 — handlers for the three preset relocation modes (below system variables, at REM, above RAMTOP).
5. Lines 1560–1600 — address patching: writes LRAM and HRAM into the machine code at addresses 17127 and 17128.
6. Lines 1700–1799 — output section reporting BASE and entry point addresses, then activating the driver via USR.
7. Lines 1800–1820 — subroutine decomposing RAM into high/low byte variables.
8. Lines 9000–9020 — diagnostic loop printing memory contents from 16626 to 16652.

Machine Code Embedding

The Z80 printer driver is embedded directly in the two REM statements at lines 1 and 3. This is a standard technique for storing machine code in BASIC programs: the bytes following the REM token are never interpreted by the BASIC interpreter and can hold arbitrary binary data. The driver is activated by calling USR 17161 or USR 17157 at lines 1785 and 1791, which are fixed absolute addresses pointing into or near the REM data area.

Relocation Modes

The program supports four relocation destinations, selected by single-character input:

Key	Destination	BASE value
R	Above RAMTOP	PEEK(16388)+256*PEEK(16389)+1
V	Below system variables	8192
T	At line 1 REM	16521 (hardcoded as BASE)
E	User-specified address	User input, validated

RAMTOP is read from system variables at addresses 16388–16389 using the standard high/low byte formula. For the “above RAMTOP” mode, the program enforces an upper limit of 32138 (code cannot run above 32767, and a margin is added), stopping with an error message if RAMTOP is too high.

Address Validation

For the user-specified (“E”) mode, the input address is checked against multiple boundaries. Addresses below 8192 or above 32767 are rejected outright. Addresses in the range 15784–17324 are also rejected, as this encompasses the system variable area. The validation loop at lines 1210–1280 re-prompts on any invalid entry, printing the offending value with an “IS AN INVALID RELOCATION ADDRESS” message before looping back to line 1225.

Address Patching Mechanism

The subroutine at lines 1800–1820 decomposes the RAM variable into HRAM (high byte, via INT(RAM/256)) and LRAM (low byte, via RAM - HRAM*256). These are then POKEd into specific locations within the machine code (addresses 17127 and 17128), effectively self-modifying the driver’s internal jump or load address so it operates correctly at the chosen relocation target. This is a classic position-dependent patching approach rather than fully position-independent code.

Entry Point Reporting

After relocation, the program prints the USR call addresses for four driver entry points, all expressed as BASE + offset:

• LCOPY at BASE+167
• LLIST at BASE+325
• CLRBUF at BASE+560
• LPRINT at BASE+171

These offsets are fixed constants baked into the listing, implying the machine code layout is fixed-size and the entry points are at known positions within it.

Linefeed Control

Lines 1000–1007 ask whether the printer needs a separate linefeed control code. If the user enters “N”, execution continues normally. If “Y”, the program skips the POKE at line 1007. The POKE at line 1007 writes 201 (the Z80 RET opcode) to address 16524, which is within the system variable / ROM call area, presumably disabling or short-circuiting a linefeed-generating routine within the driver.

Activation and Cleanup

At lines 1785 and 1791, the driver is activated by USR 17161 and USR 17157 respectively. When the “T” (REM) mode is chosen, the NEW command at line 1796 is skipped (branching to line 1799 STOP), since the driver lives in the REM area and a NEW would destroy it. For all other modes, NEW is called to clear BASIC memory, leaving only the relocated machine code resident.

Diagnostic Routine

Lines 9000–9020 provide a simple memory inspection loop, PRINTing the address and byte value for each location from 16626 to 16652. This range covers part of the system variables / driver patching area and appears to be a debugging aid left in the final listing.

Notable Idioms and Anomalies

• The use of PAUSE 180 at line 1775 gives the user approximately six seconds to read the entry point information before the screen is potentially modified.
• Line 1790 is a bare REM with no content, serving as a label target for the GOTO at line 1782 — a common ZX81 BASIC technique to annotate branch targets.
• The B$="Z" assignment at line 1050 is set but never referenced again in the visible BASIC code, suggesting it may be a remnant of an earlier version or used within the machine code context.
• The variable L assigned from USR calls at lines 1785 and 1791 captures the return value of the machine code routine but is never used, which is normal when USR is called purely for its side effects.