Italics

This program installs a custom italic character set for use in BASIC programs by POKEing 42 bytes of Z80 machine code into address 30000, then executing it with RANDOMIZE USR. The machine code (lines 30–40) copies and transforms the ROM character set from address 15616 (0x3D00) to address 30208 (0x7600), applying two right-rotation operations (RLC→RRC, i.e. CB 63 twice) to each byte to produce an italic slant effect. The resulting 768-byte block (three pages covering 96 characters) is then saved to tape as a named CODE file. After saving, the user redirects the system font pointer via POKEs to addresses 23606 and 23607 (CHARS system variable) in their own program to activate the italic set at any load address.

---

Program Analysis

Program Structure

The program is a one-shot utility that builds, executes, and saves a machine-code font transformer. Its flow is strictly linear:

1. Lines 1–5: REM documentation block explaining usage and the CHARS system variable POKEs.
2. Line 20: LIST — displays the program listing so the user can read the instructions before anything runs.
3. Line 30: Loads 42 bytes of Z80 machine code into RAM at address 30000.
4. Line 40: The DATA statement containing those 42 bytes.
5. Line 50: RANDOMIZE USR 30000 — calls the machine code routine.
6. Lines 60–80: Asks for a filename, then saves the generated font block.
7. Lines 90–100: Rewind prompt and VERIFY.
8. Lines 110–120: Completion beep and NEW to clean up.

Machine Code Routine (address 30000)

The 42 data bytes disassemble to a Z80 routine that copies the ROM character set and applies a bit-rotation to each byte to produce an italic effect:

Offset	Bytes (hex)	Mnemonic	Notes
0	21 00 3D	LD HL,3D00h	Source: ROM charset (char 32 = space)
3	11 00 76	LD DE,7600h	Destination: 30208 decimal
6	01 00 03	LD BC,0300h	768 bytes = 96 characters × 8 rows
9	ED B0	LDIR	Block copy ROM font to RAM
11	21 00 76	LD HL,7600h	Point to start of copied font
14	0E 60	LD C,60h	Outer loop: 96 characters
16	06 02	LD B,02h	Inner loop A: 2 rows per pass (top portion)
18	7E	LD A,(HL)	Load byte
19	CB 3F	SRL A	Shift right logical (italic slant)
21	CB 3F	SRL A	Second right shift
23	77	LD (HL),A	Write back
24	23	INC HL	Next row
25	10 F7	DJNZ 18	Loop B times
27	06 04	LD B,04h	Inner loop B: 4 rows (middle)
29	7E	LD A,(HL)	Load byte
30	CB 3F	SRL A	Single right shift
32	77	LD (HL),A	Write back
33	23	INC HL	Next row
34	10 F9	DJNZ 29	Loop B times
36	23	INC HL	Skip remaining 2 rows (bottom = unshifted)
37	23	INC HL
38	0D	DEC C	Decrement character counter
39	C8	RET Z	Return when all 96 chars processed
40	18 E6	JR 14	Loop back to outer character loop

The italic effect is achieved by dividing each 8-row character bitmap into three zones: the top 2 rows are shifted right by 2 bits (SRL twice), the middle 4 rows by 1 bit, and the bottom 2 rows are left unchanged. This creates a stepped rightward lean that mimics italic slant when rendered on screen.

Font Relocation and the CHARS System Variable

The generated font is saved as a raw CODE block of 768 bytes starting at 30208 (0x7600). To use it, the host program must update the CHARS system variable at addresses 23606–23607, which normally points 256 bytes before the ROM font. The REM in line 5 gives the correct formula:

• POKE 23606, addr - 256*INT(addr/256) — low byte of (addr − 256)
• POKE 23607, INT(addr/256) — high byte

Because CHARS is indexed from character 0 (not character 32), the system subtracts 256 bytes (32 chars × 8 rows) from the supplied address, so the caller must account for this offset.

Key BASIC Idioms and Notable Techniques

• LINE N$ in the INPUT statement (line 60) accepts the full input as a string without string-quote processing, allowing filenames with spaces or special characters.
• RANDOMIZE USR 30000 is the standard idiom for calling machine code; the return value from the routine (0 from RET Z) is discarded harmlessly.
• NEW at line 120 clears both the BASIC program and variables after saving, leaving a clean slate — appropriate for a utility intended to run exactly once.
• The LIST at line 20 serves as an in-program documentation display, ensuring the user reads the REM instructions before any code executes.

Anomalies and Observations

• The outer loop counter in C is loaded as 0x60 (96), covering the printable ASCII range (characters 32–127), which matches the 768-byte save length exactly.
• The bottom 2 rows of each character are skipped entirely (two INC HL instructions), so descenders and baseline pixels are not shifted — this preserves legibility at the cost of a slight discontinuity at the character baseline.
• The routine does not preserve registers, but since it is called via USR and returns with RET Z after the loop naturally exhausts, this is benign.