Programmable Sign Board Operating System Calls

Operating system calls are made through the Trap function (Assembly: TRAP opcode: 50). The second byte specifies which operating system function is called. The current calls are as follows:

Assembly Machine Code Arguments Return Values Notes

TRAP SCROLL_LEFT 50 01 none none Scroll the screen left one pixel.

TRAP SCROLL_RIGHT 50 02 none none Scroll the screen right one pixel.

TRAP DRAW_CHAR 50 03
Reg B: character index. Characters assumed to be 16 pixels high by 8 pixels wide. First character is referenced with a value of 0001 in Reg B.
Reg C: x position at which to begin drawing character.
Color is stored in memory. (27F4: red; 27F6: green; 27F8: blue)
Base address of character set is stored in memory at 27F2. none Draw the character (from character set loaded into memory) specified in Reg B at the position specified in Reg C in the color specified in memory.
First character in character set is referenced with the value 1 in Reg B.

TRAP CLEAR_SCREEN 50 04 none none Clear all pixels on the screen (set them all to black).

TRAP FILL_SCREEN 50 05 Color is stored in memory. (27F4: red; 27F6: green; 27F8: blue) none Fill the screen with a specified color.

TRAP INVERT_SCREEN 50 06 none none Invert the colors on the screen by XORing all of the bytes in the screen memory with FF. Calling this trap twice will always return the screen to its original state.

TRAP COLOR_SWAP 50 08 Colors are stored in memory.
Color1: 27F4: red; 27F6: green; 27F8: blue
Color2: 27F5: red; 27F7: green; 27F9: blue none Replace all occurences on the screen of Color1 with Color2.

TRAP COLOR_SWAP2 50 09 Colors are stored in memory.
Color1: 27F4: red; 27F6: green; 27F8: blue
Color2: 27F5: red; 27F7: green; 27F9: blue none Replace all occurences on the screen of Color2 with Color1.

TRAP DRAW_OFF 50 0A none none Suspend redrawing of the screen after trap calls.

TRAP DRAW_ON 50 0B none none Resume redrawing of the screen after trap calls including this one.

TRAP LOAD_COLORS 50 0C
Reg A: address of 6 bytes of color data.
Draw Color: 27F4: red; 27F6: green; 27F8: blue Move 6 bytes of color data from the specified address into the Draw Color reserved memory starting at 27F4.

TRAP TRAP_GET_TIME 50 20 none Reg A: tens digit of hour.
Reg B: ones digit of hour.
Reg C: tens digit of minute.
Reg D: ones digit of minute. Get the time of day.

TRAP TRAP_TIME_TIL_TRAIN 50 21 none Reg A: number of minutes until next train. Get number of minutes until the next train.

Assembly	Machine Code	Arguments	Return Values	Notes
TRAP SCROLL_LEFT	50 01	none	none	Scroll the screen left one pixel.
TRAP SCROLL_RIGHT	50 02	none	none	Scroll the screen right one pixel.
TRAP DRAW_CHAR	50 03	Reg B: character index. Characters assumed to be 16 pixels high by 8 pixels wide. First character is referenced with a value of 0001 in Reg B. Reg C: x position at which to begin drawing character. Color is stored in memory. (27F4: red; 27F6: green; 27F8: blue) Base address of character set is stored in memory at 27F2.	none	Draw the character (from character set loaded into memory) specified in Reg B at the position specified in Reg C in the color specified in memory. First character in character set is referenced with the value 1 in Reg B.
TRAP CLEAR_SCREEN	50 04	none	none	Clear all pixels on the screen (set them all to black).
TRAP FILL_SCREEN	50 05	Color is stored in memory. (27F4: red; 27F6: green; 27F8: blue)	none	Fill the screen with a specified color.
TRAP INVERT_SCREEN	50 06	none	none	Invert the colors on the screen by XORing all of the bytes in the screen memory with FF. Calling this trap twice will always return the screen to its original state.
TRAP COLOR_SWAP	50 08	Colors are stored in memory. Color1: 27F4: red; 27F6: green; 27F8: blue Color2: 27F5: red; 27F7: green; 27F9: blue	none	Replace all occurences on the screen of Color1 with Color2.
TRAP COLOR_SWAP2	50 09	Colors are stored in memory. Color1: 27F4: red; 27F6: green; 27F8: blue Color2: 27F5: red; 27F7: green; 27F9: blue	none	Replace all occurences on the screen of Color2 with Color1.
TRAP DRAW_OFF	50 0A	none	none	Suspend redrawing of the screen after trap calls.
TRAP DRAW_ON	50 0B	none	none	Resume redrawing of the screen after trap calls including this one.
TRAP LOAD_COLORS	50 0C	Reg A: address of 6 bytes of color data.	Draw Color: 27F4: red; 27F6: green; 27F8: blue	Move 6 bytes of color data from the specified address into the Draw Color reserved memory starting at 27F4.
TRAP TRAP_GET_TIME	50 20	none	Reg A: tens digit of hour. Reg B: ones digit of hour. Reg C: tens digit of minute. Reg D: ones digit of minute.	Get the time of day.
TRAP TRAP_TIME_TIL_TRAIN	50 21	none	Reg A: number of minutes until next train.	Get number of minutes until the next train.