apple1-videocard-lib/README.md
2021-12-17 17:32:15 +01:00

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# apple1-videocard-lib
Library and demos for the "Apple-1 Graphic Card" by P-LAB,
featuring the TMS9918 Video Display Processor by Texas Instruments.
## Repo structure
```
demos/
demo/ demo program that makes use of the library
picshow/ demo program that shows a picture in bitmap mode
tetris/ a game
docs/ TMS9918 and Apple-1 manuals
kickc/ target configuration files for KickC
lib/ the library files to include in your project
tools/ some build tools
```
## Introduction
The library is a set of C routines that make very easy
to use the TMS9918 on the Apple-1. It is intended to work
with the "Apple-1 Graphic Card" board by [P-LAB]() or any
other video card that maps the TMS9918 in the `$CC00`-`$CC01`
memory range of the Apple-1.
The library is written in C with [KickC](https://gitlab.com/camelot/kickc/-/releases), a very efficient 6502 C compiler.
## Choice of the screen mode
The library supports screen modes 1 and 2 only (screen 0 and screen 3 not
being very useful). Both are 256x192 pixels but there are some differences
you should consider when evaluating which mode to use:
- Screen 1: there are 256 tiles that can be written very quickly on the
screen, but the color choice is limited to 8 consecutive tiles for each
color in the palette. It's commonly used for text applications with
limited colors.
- Screen 2: you can address every pixel on the screen with a limitation
of 2 colors per line within an 8x8 tile grid. It's commonly used for bitmapped
graphic or slow-but-colorful text.
Both screen modes support 32 sprites.
### Working with screen 1
Some example code:
```c
// *** first we set the SCREEN 1 mode
tms_init_regs(SCREEN1_TABLE); // initializes the registers with SCREEN 1 precalculated values
screen1_prepare(); // prepares the screen to be used, loads a useful 8x8 ASCII font
tms_set_color(COLOR_VIOLET); // sets border color to violet (see tms9918.h for the list of all colors)
// *** then we can use it
screen1_cls(); // clears the screen
screen1_putc('A'); // writes the character "A"
screen1_putc(CHR_BACKSPACE); // goes back 1 character
screen1_putc('B'); // overwrites a "B" over it
// writes "A" in the bottom-right corner, causing the screen to scroll
screen1_locate(31,23);
screen1_putc("A");
// some printing
screen1_puts(CLS "Hello world"); // CLS, REVERSE_ON, REVERSE_OFF
screen1_puts(REVERSE_ON "reverse" REVERSE_OFF); // are macros defined in tms_screen1.h
screen1_puts("Line1\nLine2"); // '\n' is also supported
// simple string input from the keyboard (editing with CTRL-H is also supported)
char buffer[32];
screen1_strinput(buffer, 32);
screen1_prints("you wrote:");
screen1_prints(buffer);
```
### Working with screen 2
Some example code:
```c
// initializes the registers with SCREEN 2 precalculated values
tms_init_regs(SCREEN2_TABLE);
// sometimes two colors need to be packed into a single byte
// you can easily do that with the FG_BG() macro:
byte mycolor = FG_BG(COLOR_BLACK,COLOR_WHITE);
// prepares the screeen to be used as a bitmap with default colors black on white
screen2_init_bitmap(mycolor);
// plots a pixel in the middle of the screen
screen2_plot(128,96);
// and erases it:
screen2_plot_mode = PLOT_MODE_RESET; // PLOT_MODE_INVERT is also supported
screen2_plot(128,96);
screen2_plot_mode = PLOT_MODE_SET;
// draws a diagonal line
screen2_line(0,0,255,191);
// writes a character from the embedded FONT
byte col = FB_BG(COLOR_DARK_RED,COLOR_LIGH_YELLOW);
screen2_putc('A', 31, 23, col);
// writes a string
screen2_puts("HELLO", 16, 12, col);
// note: screen2_putc() and screen2_puts() are fast but they
// can only print characters aligned within the 8x8 grid
```
### Working with VRAM directly
Some example code:
```c
// writes the value 42 at VRAM location 8000
tms_set_vram_write_addr(8000);
TMS_WRITE_DATA_PORT(42);
// and re-reads it
tms_set_vram_read_addr(8000);
byte val = TMS_READ_DATA_PORT;
```
When using the default values that came with `SCREEN1_TABLE[]` and `SCREEN2_TABLE[]`,
VRAM is organized according the following memory map:
```c
// ZONE RANGE NAME YOU CAN USE IN C
// ===========================================================
// pattern table $0000-$17FF TMS_PATTERN_TABLE
// sprite patterns $1800-$19FF TMS_SPRITE_PATTERNS
// color table $2000-$27FF TMS_COLOR_TABLE
// name table $3800-$3AFF TMS_NAME_TABLE
// sprite attributes $3B00-$3BFF TMS_SPRITE_ATTRS
// example: writes the bitmap value 10101010 on row 3 of pattern 4
tms_set_vram_write_addr(TMS_PATTERN_TABLE+4*8+3);
TMS_WRITE_DATA_PORT(0b10101010);
```
### Working on a more low-level
#### Setting the TMS9918 registers
```c
// you can set a TMS9918 register directly with:
tms_write_reg(7, 0x1F);
// which also saves the written value to a buffer
// because the TMS does not allow to read from
// its registers (they are write-only)
byte oldvalue = tms_regs_latch[7];
```
#### Working directly with the I/O chip interface
If you want to program the VDP directly you can use the following utility functions:
```c
TMS_WRITE_CTRL_PORT(value); // writes a byte to the control port ($CC01)
TMS_WRITE_DATA_PORT(value); // writes a byte to the data port ($CC00)
byte value = TMS_READ_CTRL_PORT; // reads the status register ($CC01)
byte value = TMS_READ_DATA_PORT; // reads a byte from the data port ($CC00)
```
### Miscellaneous functions
```
tms_wait_end_of_frame(); // waits the end of video frame, for timimng or sync video updates
tms_set_blank(1); // turns on video blanking (0 restores normal view)
tms_set_external_video(1); // turns on/off external video input
tms_set_interrupt_bit(1); // enable end of frame interrupts generation
(TODO: interrupt functions)
```
### Apple-1 utility functions
There are also utility functions to interact with the Apple-1
screen and keyboard:
```c
// prints hex "F3" on the Apple-1 screen
woz_print_hex(0xF3)
// prints "A" on the Apple-1 screen
woz_putc('A');
// prints "HELLO" on the Apple-1 screen
woz_puts("HELLO");
// gets a key from the keyboard (waits for it)
byte k = apple1_getkey();
// non blocking keyboard read: do something until RETURN is hit
while(1) {
if(apple1_iskeypressed()) {
if(apple1_readkey()==0x0d) break;
}
else do_something_else();
}
```
### Building the source code
To link the library, simply `#include` the `tms9918.h` file
in your C sources. The recommended way is to add the KickC
command line switch `-includedir=thisrepo/lib` to your
build script and then include the file with `#include <tms9918.h>`.
The the `tools/` directory contains a simple `build.bat` script
example (for Windows) that you can customize to your needs.
### Setting a machine target
There are three configurations you can target with
the switches `-t target -targetdir thisrepo/kickc`
of the KickC compiler:
- `apple1`
- `apple1_jukebox`
- `vic20`
#### Target "apple1"
With this target, the compiled program will start at `$0280` in
the free RAM of the Apple-1 (please make sure you have enough RAM).
(TODO: add reference to `hexdump.js`)
#### Target "apple1_jukebox"
This target is for expansion cards that provide a ROM storage in
the range `$4000`-`$7FFF`, as:
- the "CodeTank" EEPROM daughterboard of the "Apple-1 Graphic Card"
- "Juke-Box Card" FLASH
In this target configuration, the program is split into two segments:
- the `Code` that resides in ROM at `$4000`
- the `Data` that resides in RAM at `$0280`
The split is required because the program needs to write on the `Data` segment (e.g. when changing the value of a variable).
The only issue is that the "Data" segment needs to be initialized
with the correct startup values (for example, the value that
a global `int` variable takes before it's used).
The "Data" initialization needs to be done manually in the C program
by explicitly calling `apple1_eprom_init()` in `main()`. The function
will copy the ROM portion $7582-$7FFF into the "Data" segment at $0280-$0FFF.
The initialization values in the ROM range $7582-$7FFF are generated with
the build script `mkeprom.js` which creates a fixed-length 16K binary file
to be put on the EEPROM/FLASH.
Below is a recap of the memory map for this target:
$0000-$00FF zero page: holds some C program variables
$0280-$0FFF RAM: C program "Data" segment
$4000-$7581 ROM: C program "Code" segment
$7582-$7FFF ROM: C program "Data" segment (startup values)
#### Target "vic20"
This target has been used during the development of the library
where a custom made VIC-20 emulator was interfaced with
an emulated TMS9918, thus allowing running tests when
the real Apple-1 machine was not available.