Thread: [Tutorial] GBA ASM Programming
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Old January 6th, 2011 (6:21 PM). Edited January 6th, 2011 by Xenesis.
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Xenesis Xenesis is offline
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Using shifts to clear bits has it's advantages. Mainly because to and something you need to load a value into a register and then and between two registers. Using bitshifts to cut off bits you don't care about is often simpler because it requires only one register.

Basically, as a consequence of the processor design there's no function for AND rX, Value, just AND rX, rY

Here's a summary of all the ALU operations from gbatek:

THUMB.4: ALU operations

Opcode Format

  Bit    Expl.
  15-10  Must be 010000b for this type of instructions
  9-6    Opcode (0-Fh)
           0: AND Rd,Rs     ;AND logical       Rd = Rd AND Rs
           1: EOR Rd,Rs     ;XOR logical       Rd = Rd XOR Rs
           2: LSL Rd,Rs     ;log. shift left   Rd = Rd << (Rs AND 0FFh)
           3: LSR Rd,Rs     ;log. shift right  Rd = Rd >> (Rs AND 0FFh)
           4: ASR Rd,Rs     ;arit shift right  Rd = Rd SAR (Rs AND 0FFh)
           5: ADC Rd,Rs     ;add with carry    Rd = Rd + Rs + Cy
           6: SBC Rd,Rs     ;sub with carry    Rd = Rd - Rs - NOT Cy
           7: ROR Rd,Rs     ;rotate right      Rd = Rd ROR (Rs AND 0FFh)
           8: TST Rd,Rs     ;test            Void = Rd AND Rs
           9: NEG Rd,Rs     ;negate            Rd = 0 - Rs
           A: CMP Rd,Rs     ;compare         Void = Rd - Rs
           B: CMN Rd,Rs     ;     Void = Rd + Rs
           C: ORR Rd,Rs     ;OR logical        Rd = Rd OR Rs
           D: MUL Rd,Rs     ;multiply          Rd = Rd * Rs
           E: BIC Rd,Rs     ;bit clear         Rd = Rd AND NOT Rs
           F: MVN Rd,Rs     ;not               Rd = NOT Rs
  5-3    Rs - Source Register       (R0..R7)
  2-0    Rd - Destination Register  (R0..R7)
As you can see, they only work between two registers (with the exception of the NOT/NEG operations).
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