This program displays a “MERRY CHRISTMAS” greeting using a machine code routine embedded in a REM statement at line 1. The machine code, stored from address 16514 onward, is a Z80 routine that manipulates the display — it uses register pairs and relative jumps (DJNZ loops) to write directly to screen memory. Lines 3 and 5 use POKE 16515 to modify a byte within the REM/machine code area itself, effectively self-modifying the routine’s behaviour between two calls via RAND USR 16514. The PRINT AT statement at line 8 positions the greeting text at row 10, column 8, and the GOTO 2 loop continuously cycles the display effect.
Program Analysis
Program Structure
The program is short but layered. Line 1 holds a REM statement whose body is raw Z80 machine code. Lines 2–6 form the main execution loop: call the machine code routine, poke a new value into it, call it again with the modified byte, then repeat forever via GOTO 2. Line 7 is a SAVE command that would save the program. Line 8 prints the greeting, and line 9 loops back — though line 8 and 9 are never reached by the main loop at line 6, suggesting the program was perhaps edited or the loop is intended to cycle only through lines 2–6.
Machine Code in the REM Statement
On the ZX81/TS1000, a REM statement’s text bytes begin at address 16514 (the byte immediately after the line-length and REM token). The bytes stored there are:
| Offset | Hex | Z80 Mnemonic | Notes |
|---|---|---|---|
| 0 | 3E 86 | LD A, 86h | Load accumulator with 134 |
| 2 | 2A 0C 40 | LD HL, (400Ch) | Load HL from display address area |
| 5 | 06 20 | LD B, 32 | Counter for 32 bytes |
| 7 | 23 | INC HL | Advance pointer |
| 8 | 77 | LD (HL), A | Write A to screen memory |
| 9 | 10 FC | DJNZ -4 | Loop 32 times |
| 11 | 06 14 | LD B, 20 | Counter for 20 rows |
| 13 | 23 23 | INC HL / INC HL | Skip two bytes |
| 15 | 77 | LD (HL), A | Write pattern |
| 16 | 11 1F 00 | LD DE, 31 | Row stride |
| 19 | 19 | ADD HL, DE | Advance by one row |
| 20 | 77 | LD (HL), A | Write pattern |
| 21 | 10 F6 | DJNZ -10 | Loop 20 rows |
| 23 | 23 | INC HL | |
| 24 | 06 20 | LD B, 32 | Counter for 32 bytes |
| 26 | 23 | INC HL | |
| 27 | 77 | LD (HL), A | Write pattern |
| 28 | 10 FC | DJNZ -4 | Loop 32 times |
| 30 | C9 | RET | Return to BASIC |
The routine draws a rectangular border pattern around the screen by writing the value in the accumulator to a run of display file bytes. The first call (line 2) uses whatever byte is currently at offset 1 of the REM (initially 86h = 134, a ZX81 graphics character). Line 3 then POKEs address 16515 (offset 1, the immediate operand of LD A, n) to 6, changing the character written. Line 4 calls the routine again with this new value, creating an animation toggle effect.
Self-Modification Technique
The key idiom here is patching the machine code’s own immediate operand through POKE 16515. Because the LD A, n instruction’s operand sits at a known, fixed address within the REM statement, lines 3 and 5 alternate the value between 6 and 134 (decimal), causing the border pattern to flash between two different graphic characters on each loop iteration. This is a classic ZX81 self-modifying code pattern used to squeeze animation out of minimal ROM calls.
Key BASIC Idioms
RAND USR 16514— the standard ZX81 idiom for calling a machine code address.RANDOMIZE USRdiscards the return value without error.POKE 16515, n— directly modifies the second byte of the REM statement (the operand ofLD A, n), achieving self-modification from BASIC.GOTO 2at line 6 creates an infinite animation loop over lines 2–5.
Unreachable Lines
Lines 8 and 9 (PRINT AT 10,8;"MERRY CHRISTMAS" and GOTO 2) are never reached during normal execution because line 6 unconditionally jumps back to line 2. These lines may be a remnant of an earlier version where the loop included a print step, or they may have been intended as a fallback that was later superseded by the machine code display routine.
Notable Observations
- The machine code references address
400Ch, which on the ZX81 falls within the display file area — confirming the routine writes directly to video RAM. - The two alternating values, 6 and 134 (= 6 + 128), are a normal character and its inverse-video equivalent, producing a blinking border effect.
- The tight DJNZ loops make the routine fast enough to produce visible flicker animation without any additional PAUSE or delay.
Content
Image Gallery
Source Code
1 REM E
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Merry Christmas Message
This file is part of Timex Sinclair Public Domain Library Tape 1002
. Download the collection to get this file.
This program displays a “MERRY CHRISTMAS” greeting using a machine code routine embedded in a REM statement at line 1. The machine code, stored from address 16514 onward, is a Z80 routine that manipulates the display — it uses register pairs and relative jumps (DJNZ loops) to write directly to screen memory. Lines 3 and 5 use POKE 16515 to modify a byte within the REM/machine code area itself, effectively self-modifying the routine’s behaviour between two calls via RAND USR 16514. The PRINT AT statement at line 8 positions the greeting text at row 10, column 8, and the GOTO 2 loop continuously cycles the display effect.
Program Analysis
Program Structure
The program is short but layered. Line 1 holds a REM statement whose body is raw Z80 machine code. Lines 2–6 form the main execution loop: call the machine code routine, poke a new value into it, call it again with the modified byte, then repeat forever via GOTO 2. Line 7 is a SAVE command that would save the program. Line 8 prints the greeting, and line 9 loops back — though line 8 and 9 are never reached by the main loop at line 6, suggesting the program was perhaps edited or the loop is intended to cycle only through lines 2–6.
Machine Code in the REM Statement
On the ZX81/TS1000, a REM statement’s text bytes begin at address 16514 (the byte immediately after the line-length and REM token). The bytes stored there are:
Offset Hex Z80 Mnemonic Notes 0 3E 86LD A, 86h Load accumulator with 134 2 2A 0C 40LD HL, (400Ch) Load HL from display address area 5 06 20LD B, 32 Counter for 32 bytes 7 23INC HL Advance pointer 8 77LD (HL), A Write A to screen memory 9 10 FCDJNZ -4 Loop 32 times 11 06 14LD B, 20 Counter for 20 rows 13 23 23INC HL / INC HL Skip two bytes 15 77LD (HL), A Write pattern 16 11 1F 00LD DE, 31 Row stride 19 19ADD HL, DE Advance by one row 20 77LD (HL), A Write pattern 21 10 F6DJNZ -10 Loop 20 rows 23 23INC HL 24 06 20LD B, 32 Counter for 32 bytes 26 23INC HL 27 77LD (HL), A Write pattern 28 10 FCDJNZ -4 Loop 32 times 30 C9RET Return to BASIC
The routine draws a rectangular border pattern around the screen by writing the value in the accumulator to a run of display file bytes. The first call (line 2) uses whatever byte is currently at offset 1 of the REM (initially 86h = 134, a ZX81 graphics character). Line 3 then POKEs address 16515 (offset 1, the immediate operand of LD A, n) to 6, changing the character written. Line 4 calls the routine again with this new value, creating an animation toggle effect.
Self-Modification Technique
The key idiom here is patching the machine code’s own immediate operand through POKE 16515. Because the LD A, n instruction’s operand sits at a known, fixed address within the REM statement, lines 3 and 5 alternate the value between 6 and 134 (decimal), causing the border pattern to flash between two different graphic characters on each loop iteration. This is a classic ZX81 self-modifying code pattern used to squeeze animation out of minimal ROM calls.
Key BASIC Idioms
RAND USR 16514 — the standard ZX81 idiom for calling a machine code address. RANDOMIZE USR discards the return value without error.POKE 16515, n — directly modifies the second byte of the REM statement (the operand of LD A, n), achieving self-modification from BASIC.GOTO 2 at line 6 creates an infinite animation loop over lines 2–5.
Unreachable Lines
Lines 8 and 9 (PRINT AT 10,8;"MERRY CHRISTMAS" and GOTO 2) are never reached during normal execution because line 6 unconditionally jumps back to line 2. These lines may be a remnant of an earlier version where the loop included a print step, or they may have been intended as a fallback that was later superseded by the machine code display routine.
Notable Observations
- The machine code references address
400Ch, which on the ZX81 falls within the display file area — confirming the routine writes directly to video RAM.
- The two alternating values, 6 and 134 (= 6 + 128), are a normal character and its inverse-video equivalent, producing a blinking border effect.
- The tight DJNZ loops make the routine fast enough to produce visible flicker animation without any additional PAUSE or delay.
Content
Image Gallery
Source Code
1 REM \3E\86\2A\0C\40\06\20\23\77\10\FC\06\14\23\23\77\11\1F\00\19\77\10\F6\23\06\20\23\77\10\FC\C9
2 RAND USR 16514
3 POKE 16515,6
4 RAND USR 16514
5 POKE 16515,134
6 GOTO 2
7 SAVE "1008%9"
8 PRINT AT 10,8;"MERRY CHRISTMAS"
9 GOTO 2
Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.
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A
Skip to content
Merry Christmas Message
This file is part of Timex Sinclair Public Domain Library Tape 1002
. Download the collection to get this file.
This program displays a “MERRY CHRISTMAS” greeting using a machine code routine embedded in a REM statement at line 1. The machine code, stored from address 16514 onward, is a Z80 routine that manipulates the display — it uses register pairs and relative jumps (DJNZ loops) to write directly to screen memory. Lines 3 and 5 use POKE 16515 to modify a byte within the REM/machine code area itself, effectively self-modifying the routine’s behaviour between two calls via RAND USR 16514. The PRINT AT statement at line 8 positions the greeting text at row 10, column 8, and the GOTO 2 loop continuously cycles the display effect.
Program Analysis
Program Structure
The program is short but layered. Line 1 holds a REM statement whose body is raw Z80 machine code. Lines 2–6 form the main execution loop: call the machine code routine, poke a new value into it, call it again with the modified byte, then repeat forever via GOTO 2. Line 7 is a SAVE command that would save the program. Line 8 prints the greeting, and line 9 loops back — though line 8 and 9 are never reached by the main loop at line 6, suggesting the program was perhaps edited or the loop is intended to cycle only through lines 2–6.
Machine Code in the REM Statement
On the ZX81/TS1000, a REM statement’s text bytes begin at address 16514 (the byte immediately after the line-length and REM token). The bytes stored there are:
Offset Hex Z80 Mnemonic Notes 0 3E 86LD A, 86h Load accumulator with 134 2 2A 0C 40LD HL, (400Ch) Load HL from display address area 5 06 20LD B, 32 Counter for 32 bytes 7 23INC HL Advance pointer 8 77LD (HL), A Write A to screen memory 9 10 FCDJNZ -4 Loop 32 times 11 06 14LD B, 20 Counter for 20 rows 13 23 23INC HL / INC HL Skip two bytes 15 77LD (HL), A Write pattern 16 11 1F 00LD DE, 31 Row stride 19 19ADD HL, DE Advance by one row 20 77LD (HL), A Write pattern 21 10 F6DJNZ -10 Loop 20 rows 23 23INC HL 24 06 20LD B, 32 Counter for 32 bytes 26 23INC HL 27 77LD (HL), A Write pattern 28 10 FCDJNZ -4 Loop 32 times 30 C9RET Return to BASIC
The routine draws a rectangular border pattern around the screen by writing the value in the accumulator to a run of display file bytes. The first call (line 2) uses whatever byte is currently at offset 1 of the REM (initially 86h = 134, a ZX81 graphics character). Line 3 then POKEs address 16515 (offset 1, the immediate operand of LD A, n) to 6, changing the character written. Line 4 calls the routine again with this new value, creating an animation toggle effect.
Self-Modification Technique
The key idiom here is patching the machine code’s own immediate operand through POKE 16515. Because the LD A, n instruction’s operand sits at a known, fixed address within the REM statement, lines 3 and 5 alternate the value between 6 and 134 (decimal), causing the border pattern to flash between two different graphic characters on each loop iteration. This is a classic ZX81 self-modifying code pattern used to squeeze animation out of minimal ROM calls.
Key BASIC Idioms
RAND USR 16514 — the standard ZX81 idiom for calling a machine code address. RANDOMIZE USR discards the return value without error.POKE 16515, n — directly modifies the second byte of the REM statement (the operand of LD A, n), achieving self-modification from BASIC.GOTO 2 at line 6 creates an infinite animation loop over lines 2–5.
Unreachable Lines
Lines 8 and 9 (PRINT AT 10,8;"MERRY CHRISTMAS" and GOTO 2) are never reached during normal execution because line 6 unconditionally jumps back to line 2. These lines may be a remnant of an earlier version where the loop included a print step, or they may have been intended as a fallback that was later superseded by the machine code display routine.
Notable Observations
- The machine code references address
400Ch, which on the ZX81 falls within the display file area — confirming the routine writes directly to video RAM.
- The two alternating values, 6 and 134 (= 6 + 128), are a normal character and its inverse-video equivalent, producing a blinking border effect.
- The tight DJNZ loops make the routine fast enough to produce visible flicker animation without any additional PAUSE or delay.
Content
Image Gallery
Source Code
1 REM \3E\86\2A\0C\40\06\20\23\77\10\FC\06\14\23\23\77\11\1F\00\19\77\10\F6\23\06\20\23\77\10\FC\C9
2 RAND USR 16514
3 POKE 16515,6
4 RAND USR 16514
5 POKE 16515,134
6 GOTO 2
7 SAVE "1008%9"
8 PRINT AT 10,8;"MERRY CHRISTMAS"
9 GOTO 2
Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.
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Merry Christmas Message
This file is part of Timex Sinclair Public Domain Library Tape 1002
. Download the collection to get this file.
This program displays a “MERRY CHRISTMAS” greeting using a machine code routine embedded in a REM statement at line 1. The machine code, stored from address 16514 onward, is a Z80 routine that manipulates the display — it uses register pairs and relative jumps (DJNZ loops) to write directly to screen memory. Lines 3 and 5 use POKE 16515 to modify a byte within the REM/machine code area itself, effectively self-modifying the routine’s behaviour between two calls via RAND USR 16514. The PRINT AT statement at line 8 positions the greeting text at row 10, column 8, and the GOTO 2 loop continuously cycles the display effect.
Program Analysis
Program Structure
The program is short but layered. Line 1 holds a REM statement whose body is raw Z80 machine code. Lines 2–6 form the main execution loop: call the machine code routine, poke a new value into it, call it again with the modified byte, then repeat forever via GOTO 2. Line 7 is a SAVE command that would save the program. Line 8 prints the greeting, and line 9 loops back — though line 8 and 9 are never reached by the main loop at line 6, suggesting the program was perhaps edited or the loop is intended to cycle only through lines 2–6.
Machine Code in the REM Statement
On the ZX81/TS1000, a REM statement’s text bytes begin at address 16514 (the byte immediately after the line-length and REM token). The bytes stored there are:
Offset Hex Z80 Mnemonic Notes 0 3E 86LD A, 86h Load accumulator with 134 2 2A 0C 40LD HL, (400Ch) Load HL from display address area 5 06 20LD B, 32 Counter for 32 bytes 7 23INC HL Advance pointer 8 77LD (HL), A Write A to screen memory 9 10 FCDJNZ -4 Loop 32 times 11 06 14LD B, 20 Counter for 20 rows 13 23 23INC HL / INC HL Skip two bytes 15 77LD (HL), A Write pattern 16 11 1F 00LD DE, 31 Row stride 19 19ADD HL, DE Advance by one row 20 77LD (HL), A Write pattern 21 10 F6DJNZ -10 Loop 20 rows 23 23INC HL 24 06 20LD B, 32 Counter for 32 bytes 26 23INC HL 27 77LD (HL), A Write pattern 28 10 FCDJNZ -4 Loop 32 times 30 C9RET Return to BASIC
The routine draws a rectangular border pattern around the screen by writing the value in the accumulator to a run of display file bytes. The first call (line 2) uses whatever byte is currently at offset 1 of the REM (initially 86h = 134, a ZX81 graphics character). Line 3 then POKEs address 16515 (offset 1, the immediate operand of LD A, n) to 6, changing the character written. Line 4 calls the routine again with this new value, creating an animation toggle effect.
Self-Modification Technique
The key idiom here is patching the machine code’s own immediate operand through POKE 16515. Because the LD A, n instruction’s operand sits at a known, fixed address within the REM statement, lines 3 and 5 alternate the value between 6 and 134 (decimal), causing the border pattern to flash between two different graphic characters on each loop iteration. This is a classic ZX81 self-modifying code pattern used to squeeze animation out of minimal ROM calls.
Key BASIC Idioms
RAND USR 16514 — the standard ZX81 idiom for calling a machine code address. RANDOMIZE USR discards the return value without error.POKE 16515, n — directly modifies the second byte of the REM statement (the operand of LD A, n), achieving self-modification from BASIC.GOTO 2 at line 6 creates an infinite animation loop over lines 2–5.
Unreachable Lines
Lines 8 and 9 (PRINT AT 10,8;"MERRY CHRISTMAS" and GOTO 2) are never reached during normal execution because line 6 unconditionally jumps back to line 2. These lines may be a remnant of an earlier version where the loop included a print step, or they may have been intended as a fallback that was later superseded by the machine code display routine.
Notable Observations
- The machine code references address
400Ch, which on the ZX81 falls within the display file area — confirming the routine writes directly to video RAM.
- The two alternating values, 6 and 134 (= 6 + 128), are a normal character and its inverse-video equivalent, producing a blinking border effect.
- The tight DJNZ loops make the routine fast enough to produce visible flicker animation without any additional PAUSE or delay.
Content
Image Gallery
Source Code
1 REM \3E\86\2A\0C\40\06\20\23\77\10\FC\06\14\23\23\77\11\1F\00\19\77\10\F6\23\06\20\23\77\10\FC\C9
2 RAND USR 16514
3 POKE 16515,6
4 RAND USR 16514
5 POKE 16515,134
6 GOTO 2
7 SAVE "1008%9"
8 PRINT AT 10,8;"MERRY CHRISTMAS"
9 GOTO 2
Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.
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\F6\FC\C9
2 RAND USR 16514
3 POKE 16515,6
4 RAND USR 16514
5 POKE 16515,134
6 GOTO 2
7 SAVE "1008%9"
8 PRINT AT 10,8;"MERRY CHRISTMAS"
9 GOTO 2Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.