OpC_30 3F - Nakazoto/CenturionComputer GitHub Wiki

3x Extended Addressing

If an odd register is specified for a 3x instruction, the operation is on memory. Note that this is still very much WIP.

For a register specifier of 1, the effective address is the direct address of the word following. For other odd registers, the effective address is indexed from the relative 16 bit register.

Register Nibble	Meaning	Example	Description
1	Direct	32 12 1234	Clear word at address 1234 to 2
3	Indexed B	32 32 1234	Clear word at address 1234 + B to 2
5	Indexed X	32 52 1234	Clear word at address 1234 + X to 2
7	Indexed Y	32 72 1234	Clear word at address 1234 + Y to 2
9	Indexed Z	32 92 1234	Clear word at address 1234 + Z to 2
b	Indexed S	32 B2 1234	Clear word at address 1234 + S to 2
d	Indexed C	32 D2 1234	Possibly illegal. Clear word at address 1234 + C to 2
f	Indexed P	32 F2 1234	Clear word at address 1234 + P to 2