The deMon is a simple piece of software that boots up the reZet80 PIONEER and enables you to start simple tasks.
After reset the start of RAM and the amount of RAM in KiB will be displayed (shall be "3C0001" for a 1 KiB and "380002" for a 2 KiB system). The first four hexadecimal digits specify the RAM start address and the last two digits show the amount of RAM in KiB. Please check the reZet80 PIONEER address decoding specs for details on the address space.
You can enter no more than 6 digits. All following digits are ignored.
The BACK key moves you one digit to the left.
Pressing ESC brings you back to the first display position.
Please note that digits you have already entered will not be deleted but can be overwritten.
Press ENTER when you have finished entering your command.
The list of available commands:
- "0XXXX": jump to specified address ("00000" for soft reset)
- "1ZYYWW": output byte to 16-bit I/O port
- "EXXXX": examine/modify memory from specified address on
Please make sure that the address XXXX and the parameters WW, YY and Z you enter are valid because no plausibility checks are done in the deMon.
In examine/modify mode you can examine the content of ROM and RAM but you can only modify RAM. The address you supplied and the value of that memory location is displayed. Entering "E0000" for example will display "0000F3" (F3 is the opcode of the "di" instruction that is the first instruction in the ROM at address 0000). Press ENTER to move to the next memory location. In our example "00013E" is then displayed. Press ENTER once more to see "0002FF" (3EFF is the instruction "ld a, ffh" that follows "di" in the ROM).
You can change the content of a RAM address by entering a new value before pressing ENTER. As you type the new value is displayed replacing the old one. You can enter 2 digits at most (00-FF). All additional digits are ignored.
Use the BACK key to correct your entry.
ESC quits examine/modify mode and you are taken back to the first display position.
Enter "167F00" to output "0" to the leftmost display position, "167F3F" for the 7-segment display. This is just an example to explain the output command. "00" or "3F" is the byte to output (WW). "7F" (YY) is the 8-bit I/O port. 70-7F can be used for the 6-digit display (see I/O decoding specs). 6 (Z) is the leftmost display digit, 1 the rightmost.
Usually one card uses one I/O port in the reZet80 PIONEER's 8-bit I/O space. Additionally a second 8-bit value is used to address one out of 256 possible I/O addresses per I/O port (1 to 6 for the display).
The display will change to "067F00" or "067F3F" after the command has been executed. Press ESC to restore the display to the command you have entered and move back to the first display position.
The SHIFT key is not used for now.
The code has been optimized for size and, if possible, for speed, too.
It makes use of both register sets and the least significant half of "ix" (the undocumented "ixl" register). "ixh" and "iy" including "iyl" and "iyh" are not used in the code (I keep a reserve for the next release).
Maskable interrupts are run in mode 1 so the "i" register is used, too.
The deMon occupies only a small amount of the 2 KiB ROM. Depending on your hardware configuration at least 1639 bytes are free for your enhancements.