Pocket65

Handheld PC based on the original 6502 CPU that fits into a pocket.

Features

• 8 digit multiplexed 7-segment display,
• 24 button keyboard,
• 2 KB of ROM and 2 KB of RAM,
• built-in Li-Po power supply and charger,
• monitor FW that allows memory edit and running user programs,
• expansion slot with 3 chip selects (up to 24 KB of external I/O and/or memory).

Memory map

Start	End	Description
0x0000	0x07FF	RAM
0x0800	0x1FFF	RAM aliased
0x2000	0x3FFF	External 1
0x4000	0x5FFF	External 2
0x6000	0x7FFF	External 3
0x8000	0x9FFF	Screen
0xA000	0xBFFF	Keyboard
0xC000	0xDFFF	NMI ack
0xE000	0xF7FF	ROM aliased
0xF800	0xFFFF	ROM

Monitor how-to

Startup

First, welcome splash-screen is displayed:

After one second memory test result is displayed:

On the right is a decimal number of RAM bytes found. By default it's 2048 bytes, memory can be expanded using expansion slot. If an error occurs during memory test, error screen is presented and the system is halted:

After 2 seconds computer proceeds to the main mode of operation:

Memory edit

In the main mode and byte modification selected (dot near memory value) memory modification is possible. To modify memory press a key 0 to F. The old value is then shifted 4 binary places to the left and new nibble is inserted at the youngest position. Previous 4 oldest bits are lost.

Example - entering 0xBA at current address:

We start with our memory cell selected (in this case at the address 0x0200):

press B button to enter 0xB:

then press A button to finish entering the byte:

Done!

Address selection

The address can be modified in two ways:

INC/DEC buttons

To select next/prev memory cell press INC/DEC button. The address will be incremented/decremented by one.

Entering new address

Big leaps through memory space are not very convenient using incrementation/decrementation. You can change the address very similar to changing memory value. To enter address edit mode pres SEL key. Dot will appear next to address to confirm mode selection:

In this mode address can be modified the same way memory value is performed - when key 0-F is pressed the old address is shifted 4 bits to the left and new value is inserted at the youngest position.

To exit address edit mode (and return to the memory value edit mode) press SEL key.

Executing user program

To start user program provides its entry point in the address field and press GO button.

F1 key - auto increment mode

F1 key toggles auto-increment mode. When on it's indicated by dots enabled next to both address and value:

When this mode is active when the whole byte is entered (i.e. two key 0-F presses) address will be incremented automatically. This mode is very useful for binary program listing input, as no additional key press between bytes is needed.

F2 key - memory clear

F2 key clears user memory - sets it to all zeroes. Memory locations from 0x0000 to 0x000CF and from 0x2000 to RAM end are cleared.

F3 key - memory copy

F3 key copies length bytes from source to destination. Destination is selected as the current address. After pressing F3 user is prompted for source address:

selection is confirmed with GO key. After that user is prompted for length:

selection is again confirmed with GO key. Memory is then copied and computer returns to the main screen.

F4 key - jump to the external ROM

F4 key allows for quick enter to the external ROM placed at address 0x6000 (external chip select #3). To detect if the external ROM is present monitor checks for a magic value at address 0x6000. Expected value is 0x4B41 (little-endian). If this check is successful jump to external ROM at address 0x6002 is performed. Otherwise an error message is displayed:

Programming

User program examples are provided in fw/example directory, along with user program template, BIOS calls user library and Makefile.

User is free to use all memory, exluding:

Start	End	Description
0x00D0	0x00FF	ROM ZP area
0x0100	0x01FF	HW stack
0x8000	0xDFFF	HW handled by ROM

Interrupts

Interrupt vectors are located in ROM area, so they cannot be changed by application code. Monitor can handle user IRQ and NMI handlers - there are two system calls to register user interrupt handler

• svc_irq_register register the IRQ handler,
• svc_nmi_register register the NMI handler.

The handler address is passed in X (low byte) and Y (high byte) registers.

Use RTS instruction to return from the handler insted of RTI - monitor handles return from the interrupt part.

System calls

Convention

When X, Y registers are used to pass 16-bit value, X holds low byte and Y holds high byte.

svc_reset

Perform warm reset.

• Arguments: none.
• Returns: none.
• Destroys: N/A.

svc_irq_register

Register user IRQ handler and enables interrupts.

• Arguments: X, Y - handler address.
• Returns: none.
• Destroys: A.

svc_nmi_register

Register user NMI handler.

• Arguments: X, Y - handler address.
• Returns: none.
• Destroys: A.

svc_get_screen_ptr

Return a pointer to the screen frame buffer to allow direct screen manipulation.

• Arguments: nome.
• Returns: X, Y - address.
• Destroys: none.

svc_putch

Displays hex character on the screen on position X.

• Arguments: A - character, X - position
• Returns: none.
• Destroys: Y.

svc_putbyte

Display a hex byte on the screen on position X. X is incremented to point on the next free screen posion.

• Arguments: A - byte, X - position
• Returns: none.
• Destroys: none.

svc_dot_enable

Enable dot on the position X.

• Arguments: X - position
• Returns: none.
• Destroys: A.

svc_dot_disable

Disable dot on the position X.

• Arguments: X - position
• Returns: none.
• Destroys: A.

svc_clear_screen

Clear screen.

• Arguments: none.
• Returns: none.
• Destroys: A, X.

svc_prinx_u16

Print 16-bit number in registers X, Y as hexadecimal number justified to the right.

• Arguments: X, Y - number.
• Returns: none.
• Destroys: A, X.

svc_prind_u16

Print 16-bit number in registers X, Y as unsigned decimal number justified to the right.

• Arguments: X, Y - number.
• Returns: none.
• Destroys: A, X, Y.

svc_prind_s16

Print 16-bit number in registers X, Y as signed decimal number justified to the right.

• Arguments: X, Y - number.
• Returns: none.
• Destroys: A, X, Y.

svc_prind_s16

Print NULL terminated decoded 7 segment string from position A.

• Arguments: X, Y - pointer to the string, A - position on the screen.
• Returns: none.
• Destroys: A, X, Y.

svc_scanx_u16

Get 16-bit unsigned number from the user (hexadecimal input).

• Arguments: none.
• Returns: X, Y - number.
• Destroys: A, X, Y.

svc_scand_u16

Get 16-bit unsigned number from the user (decimal input).

• Arguments: none.
• Returns: X, Y - number.
• Destroys: A, X, Y.

svc_get_key

Get key code - block until new key is pressed.

• Arguments: none.
• Returns: A - key code.
• Destroys: none.

svc_get_key_nb

Get current key code.

• Arguments: none.
• Returns: A - key code.
• Destroys: none.

svc_get_jiffies

Get current miliseconds value (one byte).

• Arguments: none.
• Returns: A - jiffies.
• Destroys: none.

svc_get_seconds

Get current seconds value (one byte).

• Arguments: none.
• Returns: A - seconds.
• Destroys: none.

svc_msleep

Sleep for 0-255 ms.

• Arguments: A - time (ms).
• Returns: none.
• Destroys: A.

svc_sleep

Sleep for 0-255 s.

• Arguments: A - time (s).
• Returns: none.
• Destroys: A.

License

Free for non-commercial use and educational purposes. See LICENSE.md for details.