3 - 1 BME 303 Lecture 14 Return and go over Exam #1 (Exam...

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Unformatted text preview: 1 BME 303 Lecture 14 Return and go over Exam #1 (Exam answers now on Blackboard) Finish: LC-3 Instructions (Still havent had a chance to address grading issues for HW #2 & #3) 2 Instructions Operate Manipulate data directly: ADD, AND, NOT Data Movement (load and store) Move data between memory and registers: LD, LDI, LDR, LEA, ST, STI, STR Control (jump, branch, return, ) Change the sequence of instruction execution: BR, JMP/RET, JSR/JSRR, RTI, TRAP 3 Control Instructions Used to alter the sequence of instructions (by changing PC) Conditional Branches branch is taken if a specified condition is true offset is added to the PC to yield new PC else, the branch is not taken PC is not changed, points to the next sequential instruction Unconditional Branches (or Jump) always changes the PC 4 Condition Codes LC-3 has three condition code registers : N negative Z zero P positive (greater than zero) Condition code registers are set by any instruction that stores a value to a register ADD, AND, NOT, LD, LDR, LDI, LEA Only one will be set at all times Based on the last instruction that altered a register 5 Branch Instruction Branch specifies one or more condition codes If the bit specified is set, the branch is taken PC is set to the address specified in the instruction Like direct mode addressing, target address is made by adding current (pre-incremented) PC with signed PCoffset9 (IR<8:0>) If the branch is not taken, the next sequential instruction is executed 1 1 1 PCoffset9 n z p BR 0x101 p BR 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Operand(s) Opcode 6 Branch Instruction if ( (n AND N) OR (z AND Z) OR (p AND P) ) PC = PC +1 + SEXT(PCoffset9) For this case, PC = PC 254 if P is set....
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3 - 1 BME 303 Lecture 14 Return and go over Exam #1 (Exam...

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