1 / 19

The LC-3 – Chapter 5

The LC-3 – Chapter 5. COMP 2620. Data Movement Instructions. Recall that there are three types of instructions: Operate instructions Data movement instructions Control instructions We consider the data movement instructions today. Data Movement Instructions.

gurit
Télécharger la présentation

The LC-3 – Chapter 5

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The LC-3 – Chapter 5 COMP 2620 Dr. James Money COMP 2620

  2. Data Movement Instructions • Recall that there are three types of instructions: • Operate instructions • Data movement instructions • Control instructions • We consider the data movement instructions today

  3. Data Movement Instructions • Remember these types of instructions move data from registers to memory and back • For now, we will not consider the movement of data from registers to input/output devices • We consider only to/from memory to general purpose registers (GPRs)

  4. Data Movement Instructions • The process of move information from memory to a register is called a load • The process of moving information from a register to memory is called a store • In both cases, the source operand completely overwrites the destination operand

  5. Data Movement Instructions • The LC-3 has the following movement instructions: • LD – load PC relative • LDR – load base+offset • LDI – load indirect • LEA – load effective address • ST –store PC relative • STR – store base+offset • STI – store indirect

  6. Data Movement Instructions • The format of the load and store instructions is • Bits [11:9] is first operand, a register • DR = destination register • SR = source register • Depends on whether load or store

  7. Data Movement Instructions • Bits [8:0] contain the address generation bits • This encodes the information for the 16 bit address of the second operand • There are four ways to interpret this • The opcode determines which way these bits are interpreted

  8. PC-Relative Mode • The opcodes LD (0010) and ST (0011) use PC-relative addressing mode • Bits [8:0] refers to an offset of the PC address after it has been incremented • The address is computed by sign extending [8:0] to 16 bits

  9. LD (PC-Relative)

  10. ST (PC-Relative)

  11. PC-Relative Mode • Note there are only 9 bits for the relative address offset • This corresponds to -28…28-1 or -256…255 • If the data is further away, then we must use some other instruction to get to the data such as LDR or LDI

  12. PC-Relative Mode • Consider the instruction • This is LD R2,0x1AF • Assume the instruction is at 0x4018 • So 0x4019-0x51 = 0x3FC8 • Then R2=MEM[0x3FC8]

  13. PC-Relative Mode • Consider the instruction • This is ST R6,0xDA • Assume the instruction is at 0x4018 • So 0x4019+0xDA = 0x40F3 • Then MEM[0x40F3]=R6

  14. Indirect Mode • LDI (1010) and STI (1011) specify indirect addressing mode for memory references • The address is formed the same way as ST or LD by sign extending the address bits • Then the value is added to the incremented value of the PC

  15. Indirect Mode • However, this is not the address of the operand • This location contains the address of the operand! • Hence, the name indirect • You can think of the operand as a pointer to the real address

  16. LDI (Indirect)

  17. STI (Indirect)

  18. Indirect Mode • Consider the instruction • The is LDI R3,0x1CC • Assume the instruction is at 0x4A1B • So 0x4A1B-0x34 = 0x49E8 • Assume MEM[0x49E8]=0x2110 • Then R3=MEM[0x2110]

  19. Indirect Mode

More Related