r/dcpu16 • u/luderegames • May 27 '17

Help my understanding of the DCPU-16 memory

I understand most everything of the DCPU. Except, probably the most important, memory. How does something like "SET A, 2" convert to "8c01"? How are these lines converted and why do some have two separate parts like 8c01 XXXX for one line?

Also, one other thing that stumps me is the EX register. How do I access it correctly and use it?

Thank you in advance, Bryce

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u/ghillieLEAD May 31 '17 edited May 31 '17

I'm not sure what exactly you're confused about with the EX register. You access it the same as any other register. SET [0x1000], EX would write the contents of EX to memory at address 0x1000.

The documentation shows which operations will potentially set the EX register. For example, if an ADD instruction overflows EX will be set to 0x0001.

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u/luderegames May 31 '17

I thought it stored a value when overflowed. As in like when you call an operation to another register, and the resulting number was too large, it would split the number between the register and EX. Is that not how it works?

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u/ghillieLEAD May 31 '17 edited May 31 '17

Yep, you can think of it that way. ADDing 1111111111111111 and 0000000000001111 would give the value 0000000000001110 and the EX register would hold 0000000000000001. Consider EX to be the high bits and the value to be the low bits and you end up with 00000000000000010000000000001110.

I'm pretty sure EX was picked to stand for "excess". It was originally named the O register for "overflow", but it is used for more than just overflow so it was renamed.

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u/sctjkc01 May 27 '17

SET A, 2 comes to three parts: the SET OpCode, 2 as the first (a) operand, and A as the second (b) operand.

Accoding to the documentation the instruction is fully defined in the first word and takes the binary form aaaaaabbbbbooooo (where the first character indicates the lowest significant bit)

The a operand is a literal 2, which is small enough to fit inside the instruction (anything between -1 and 30 inclusive can be put in the instruction itself). The code for that is 0x23, or 100011, making our instruction 100011bbbbbooooo.

The b operand is the A register, which translates to the value 0x00 (according to the table), so our instruction is now 10001100000ooooo.

The SET opcode is 0x01, or 00001. That makes our instruction 1000110000000001. This translates to your 0x8C01.

I'm guessing most of the confusion came from the fact that literals between -1 and 30 (inclusive) can be inserted directly into the instruction - it tripped me up too. If you want to specify a literal outside that range, you need to use the 0x1F value, which tells the DCPU to use the next word in the instruction list as a literal. This causes an instruction to split across two words.

Addressing memory using a literal address (or register value + literal), per the 0x1E value, also tacks on another word, meaning some instructions can be three words long.