LOAD

Loads a value into a the given register

Loads from memory into a register

LD(PC-Relative)

Loads a value into a register, relative to when this command is run

0010 DDD PPPPPPPPP

• 0010: Load Command
• DDD: Destination register(3bits, R0-R7)
• PPPPPPPPP: Source address, relative to this command + 1

Example

Load EXACTLY what is in register R3(say R0 = 0000 0000 0000 0001), into the relative source address(Number of instructions in your code away from when the LD command is executed + 1.

0011 0000 0000 0000 ; line 1: starting address of 0x3000 to start the program

0010 011 0 0000 0010 ; line 2: LD, R3 <- offset by 2 lines(0 0000 0010)

0001 010 010 1 00001 ; line 3: ADD, R2 <- R2 + 1, Random instruction


1111 0000 0010 0101 ; line 4: halt end program, HALT

; Variables

0000 0000 0000 1111 ; line 5: Random value to store into R3

Let's inspect the LD instruction in detail now

0010 011 0 0000 0010

0010 : This is the opcode, if you want to use the LD instruction it will ALWAYS be this for the first 4

011 : This is the destination register, we are STORING a value into this register.

• In this case, 011(binary) = 3(decimal) so we are storing the value in Register 3(R3).

0 0000 0010 : This is the source address.

• This part is 9-bits wide and can also be written as 000000010 But I added spaces for organization.

• This number is used as an offset to your current location + 1 to get to the address who's value is to be stored into our given register above.

  • The + 1 is because the program counter increments by 1 after the instruction but before the next(Not 100% sure on the reason but just know that its offset + 1).

• The calculation for 0 0000 0010(binary) = 2(decimal) so our offset will be 2 + 1 = 3

• Our LD instruction was on line 2, therefore our source address, who's value we will be moving into R3, will be line 2 + 3 lines = line 5 0000 0000 0000 1111.

LDR(PC-Base + Relative)