3 ms·

Why would you favour a AOS (Array of Structures) or the SOA (Structure of Arrays), for the different architectures?

• to3m 11y ago
  The basic operation for accessing a value from a structure is to take the address of the structure, add a compile-time constant offset to it, and use that address. There's an addressing mode specifically for this in the Z80: you put the struct address in a register, and have the constant offset as part of the instruction. So you can do stuff like this: (numbers in comments are: <bytes for this instruction> <bytes total> / <cycles for this instruction> <cycles total>) <<put address in IX>> LD A,(IX+0) ; A=SELF.X 3 3 / 19 19 ADD A,(IX+2) ; A=SELF.X+SELF.DX 3 6 / 19 38 LD (IX+0),A ; SELF.X=SELF.X+SELF.DX 3 9 / 19 57 XOR A ; A=0 1 10 / 4 61 LD (IX+2),A ; SELF.DX=0 3 13 / 19 80 The 6502 doesn't have anything exactly like this. You do have an indirect addressing mode, but the offset goes in a register. So for every access, you need to have the offset into the index register. (There's no point adding the offset to the address; the offset is mandatory, so you'd then just have to load zero into the index register every time.) So the above in 6502: <<put address in $70>> LDY #2 2 2 / 2 2 LDA ($70),Y ; A=SELF.DX 2 4 / 5 7 LDY #0 2 6 / 2 9 CLC 1 7 / 2 11 ADC ($70),Y ; A=SELF.DX+SELF.X 2 9 / 5 16 STA ($70),Y ; SELF.X=SELF.X+SELF.DX 2 11 / 6 22 TYA 1 12 / 2 24 LDY #2 2 14 / 2 26 STA ($70),Y ; SELF.DX=0 2 16 / 6 32 Which is rather unwieldy, takes up more space, and is a bit slow (the 4:1 ratio I mentioned in my original past is here closer to 2:1) - even after I rearranged the code a bit so the Y value didn't need reloading so much. You also have the issue that the struct address is a 16-bit quantity, something that is inconvenient to handle on the 6502. There are no instructions that operate on 16-bit data. One thing the 6502 does have though is an addressing mode where the operand's address is a fixed 16-bit value (from the instruction) plus an 8-bit index register. So, with this in mind, what you could do is have a separate table for each struct field. So this switches things around, and instead of a runtime 16-bit address (of the struct) and a compile-time 8-bit offset (of the field), you have a compile-time 16-bit address (of the field table) and a runtime 8-bit offset (for the row in the table). Which is a perfect fit. Best of all, the indexing is free if you arrange things correctly, which is not that hard to do. Then the code would look more like this: <<put row in X>> CLC ; 1 1 / 2 2 LDA XS,X ; A = SELF.X 3 4 / 4 6 ADC DXS,X ; A = SELF.X+SELF.DX 3 7 / 4 10 STA XS,X ; SELF.X=SELF.X+SELF.DX 3 10 / 4 14 LDA #0 ; 2 12 / 2 16 STA DXS,X ; SELF.DX=0 3 15 / 4 20 And there's that 4:1 ratio again... Each object's "address" is now an 8-bit quantity (it's just an index into the tables), so much easier to deal with. And moving from one object to the next is very quick, since it's just incrementing a register. There doesn't appear to be anything analogous to this alternative approach on the Z80.