AFNT320 — text output at 320×256

AFNT320.DLL is a small library for fast text output in mode #81 (320×256, 256 colours). The font is embedded right in the DLL and is drawn by the Sprinter hardware accelerator, so a string appears on screen much faster than with CPU per-pixel rendering. It is the 320×256 counterpart of AFNT640: the two share the same interface.

The shared DLL loading rules are in the libraries overview.

Functions

#	Registers	What it does
0	—	initialise (called by libman on load)
1	—	release (called by libman on unload)
2	A	fnstyle: set the EGA palette and clear the selected screen
3	DE, IX, IY, A	aprint: print the ASCIIZ string at DE at point (IX,IY); A = background·16 + text colour
4	E	select the CPU window (0..3) used to map video memory
5	E	select the target buffer: 0=BUF0, 1=BUF1, 2=FRONT, 3=BACK

Example

        include "afnt320.inc"

        ld   hl,dll_name        ; "AFNT320.DLL",0
        ld   a,3                ; DLL — into window WIN3
        call LIBMAN.l_load
        jp   c,load_error
        ld   (handle),hl

        ; palette + screen clear
        ld   hl,(handle)
        ld   a,0                ; clear colour
        ld   b,2                ; fnstyle
        call LIBMAN.l_call

        ; print a string
        ld   hl,(handle)
        ld   de,text            ; ASCIIZ string
        ld   ix,8               ; x in pixels
        ld   iy,12              ; y in pixels
        ld   a,#1F              ; background 1 (blue), text 15 (white)
        ld   b,3                ; aprint
        call LIBMAN.l_call

text:   db  "Hello, Sprinter!",0

Walkthrough: function 2 loads the standard EGA palette and clears the screen — calling it once after loading is enough. Function 3 prints a string; the colour attribute in A works like in text mode: high nibble is the background, low nibble the character colour. The font is proportional, so there is no character grid — coordinates are given in pixels.

Selecting a buffer: BUF0/BUF1 and FRONT/BACK

In mode #81 Sprinter has two screen buffers. Function 5 chooses where to print:

• BUF0 / BUF1 — a specific physical buffer;
• FRONT / BACK — “visible now” and “hidden now”: the library reads the RGMOD register once per call and resolves the physical buffer itself.

The FRONT/BACK mode is convenient with double buffering: print into BACK, then flip the buffers — text never flickers on the visible screen. Without an explicit target the library behaves like older versions: it prints into BUF0. To get the palette installed in both buffers, call set_target(BUF0) + fnstyle, then set_target(BUF1) + fnstyle.

An invalid selector returns A=#10 (AFNT320_ERR_ARGUMENT) without changing the previous target.

The video memory window

To draw, the library temporarily maps a video memory page into one of the CPU windows — WIN1 by default. If your program occupies WIN1 itself (or the DLL was loaded there), select another window with function 4. A request that would cover the window holding the DLL or the current stack is rejected with an error, keeping the previous setting. After every call the library restores the mapped page and the interrupt state.

How it works inside

The font is stored in the DLL in an “accelerator” format: at build time a tool repacks the ordinary raster font into column-major masks that the Sprinter accelerator transfers to video memory as whole vertical columns. Empty columns are filled with a single operation; occupied ones are combined from text- and background-colour masks, and on a black background the extra pass is skipped entirely. Screen clearing also runs on the accelerator: 320 vertical fills instead of a loop over 80,000 pixels. That is where the speed comes from.