The Zoomer's directional keypad and A/B buttons are accessed by the BIOS interrupt number 16 (22 decimal), which corresponds to the keyboard interface in desktop PCs.
The "keyboard" state may be checked by means of polling or by installing a routine (similar to an interrupt handler) to be called whenever a button is pushed or the keypad is moved in a different direction.
Please note: Except where specified otherwise, all numeric values are given in hexadecimal.
| Int 16 Functions | ||
| AX | BH | Description |
| 5001 | 00 | Purpose unknown (Initialization?) |
| 5001 | 01 | Purpose unknown |
| 5002 | Read keypad state | |
| 5003 | 00 | Get keyboard handler address |
| 5003 | 01 | Set keyboard handler address |
Click on a specific function description in the above table to jump to its detailed description immediately. Other information available on this subject:
| Other information |
| Keypad Flags Byte |
| Example program (assembly language) |
| Parameters | |
| Register | Contents |
| AX | 5001 |
| BH | 00 |
| Returns | |
| BX | 0001 |
The purpose of this call is not yet known. It seems to be the first call made to the keyboard interface, prior to reading and setting the keyboard interrupt handler address.
It may be an initialization call, similar to the INT 33/AX=0 call to the mouse driver in desktop PCs.
| Parameters | |
| Register | Contents |
| AX | 5001 |
| BH | 01 |
| Returns | |
| BX | ???? |
The purpose of this call is not yet known. It seems to be the final call made to the keyboard interface, prior to reading and setting the keyboard interrupt handler address. Before calling INT 16, BL is set to 01 as well.
| Parameters | |
| Register | Contents |
| AX | 5002 |
| Returns | |
| AL | Keypad flags byte (see below) |
This function returns the state of the directional keypad and the A/B buttons, and returns a bitmapped flag byte in the AL register. AH is always set to 01.
Due to the physical construction of the directional keypad, diametrically opposing bits cannot be set (e.g., the UP flag will not be set at the same time as the DOWN flag).
| Bit Numbers | Description | |||||||
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
| X | X | C0 - Unused | ||||||
| X | 20 - Keypad Right | |||||||
| X | 10 - Keypad Down | |||||||
| X | 08 - Keypad Left | |||||||
| X | 04 - Keypad Up | |||||||
| X | 02 - "A" Button | |||||||
| X | 01 - "B" Button | |||||||
| Parameters | |
| Register | Contents |
| AX | 5003 |
| BH | 00 |
| Returns | |
| CX | Offset of current handler |
| DX | Segment of current handler |
This function returns the address of the current keyboard handler. This handler is called whenever a button is pressed or released, or whenever the directional keypad is moved in a different direction (or returned to its center position).
See the next section for information about the keyboard handler.
| Parameters | |
| Register | Contents |
| AX | 5003 |
| BH | 00 |
| CX | Offset of new handler |
| DX | Segment of new handler |
| Returns | |
| Nothing | |
This function installs a new keyboard handler. This handler is called whenever a button is pressed or released, or whenever the directional keypad is moved in a different direction (or returned to its center position).
Upon entry, the routine receives the new value of the keypad state in the AL register, and the previous state in the AH register. (See the table named "Keypad Flags Byte" for an explanation of the key flags.)
When the routine is called, the value of the Data and Extra segments is undefined. The handler routine should save the values of these registers, as well as any other registers which it uses, and restore them before exiting.
The routine must be a FAR procedure, and exit with a far return.