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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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:

Code:
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 ;neg.compare 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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