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Emulator: System Calls

This post will cover some basic I/O system calls, enough to implement a game or two, other than sound support or holotape support. The holotape calls required a little more explanation, and the sound system data is more complex than for the calls covered here.

Prerequisites

Hopefully you've read the post on running code in the emulator. It describes the easiest, fastest way to get your code running.

Console Calls

In the Clear

The CLEAR system call will remove all text and attributes from the screen, and set the cursor to top-left (0,0). 

Character I/O

The only console input system call is GETCH. It takes a byte off the stack as the blocking mode (=0 no blocking, != 0 blocking), grabs a key from the input queue and returns a 16-bit value indicating which character was pressed. If the MSB is 0 then the LSB contains the ASCII value.

There are two output calls, SETCH and PRINT. They both increment the cursor to the next character after their output is printed to screen at the current character. SETCH takes a single-byte ASCII character, whereas PRINT takes a null-terminated string starting at the location pointed to by the X register.

Cursor Control

The cursor location consists of 16-bit X and Y coordinates, within the space of the coordinate space of the text screen.

To set the cursor, call SETCURSOR with the desired X and Y coordinates on the stack. If the operation worked, the byte 0 will be on top of the stack.

To get the current position of the cursor, call GETCURSOR. The X and Y coordinates will be on the top of the stack after executing this system call.

Screen Attributes

Each character on the screen has an attribute. The current possible attributes are "dim" and "inverted." These are combined in a bit mask, where the second least-significant-bit represents the "dim" attribute, and the least-significant-bit represents "inverted."

SETATTR changes the default attribute for new characters written to screen. This does not change the attributes of any character, but any further characters will have these attributes.

SETATTRC sets the attributes on the character currently under the cursor. This does not affect the attributes of any subsequent character output.

Graphics Calls

Use GRAPHICSTART to begin a graphics session. The value in X will be used as the starting address for the framebuffer, and the byte passed on the stack will be used to set the graphics mode. See the table below for the possible values of the mode byte.

On return, the system call places a byte on the stack. If the graphics mode is available, this byte is 0. If the byte describes a non-existent mode, or the mode requires more memory than is available to the system, then the byte is non-zero.

Depths less than 8 bpp store their values packed into a byte. For example, if using the 1 bpp mode, each byte of graphics memory will contain 8 pixels.

There is currently no blitter library yet, so it's a roll-your-own situation.

Table of System Calls

GETCH

Polls the keyboard for a key press, and returns the 16-bit value, ASCII in LSB if MSB is 0

Blocking mode

0 if no character pressed and non-blocking, otherwise the key pressed

SETCH

Prints a character at the text cursor position and increments the cursor

Character

-

PRINT

Prints a null-terminated string from X at the text cursor position and increments the cursor accordingly

Null-terminated string starting at X

-

SETCURSOR

Sets the current text cursor position

16-bit X and Y coordinates for the cursor

0 if valid

SETATTR

Sets the current text attribute flags

16-bit field for text attribute flags

-

SETATTRC

Sets the text attribute of the character under the cursor

16-bit field for text attribute flags

-

CLEAR

Clears the screen of text and attributes, and sets the cursor to 0,0

-

-

GETCURSOR

Gets the current text cursor position

-

16-bit X and Y coordinates for the cursor


GRAPHICSTART

Begins a graphics session in the provided mode, with the framebuffer at the address in X. Can be called at any time, and will override the current graphics session, if any. Can be used for double buffering or animation.

Graphics mode

0 on success, error otherwise

GRAPHICEND

Ends all graphics sessions, returning to the text display

-

-

Graphics Mode Table

Bits

Name

Possible values

6-4

Colour depth

000 = 1bpp

001 = 2bpp

010 = 4bpp

011 = 8bpp

1xx = reserved

3

Border enabled

0 = black border

1 = green border

(if applicable)

2-0

Resolution

000 = 120x80

001 = 192x128

010 = 240x160

011 = 320x240

100 = 480x320

101 = reserved

110 = reserved

111 = reserved

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