CS25410 Memory Machine Code

Common types of non-rotating memory (1) RAMRandom Access Memory In reality, read/write memory This is usually volatile, meaning it forgets its contents when the power is turned off Magnetic core retains memory without power ROMRead Only Memory This is non-volatile, and is written onto the device permanently in the factory

Common types of memory (2) PROMProgrammable Read Only Memory This is non-volatile, and is written once by the user. It cannot be written to a second time EPROMErasable PROM This is a type of PROM that can be erased and then re-programmed. Erasing is usually done by exposure to UV light

Common types of memory (3) EEPROMElectrically Erasable PROM This is another type of PROM that can be erased and then re-programmed. This time, erasing can be done electronically in-circuit, by applying a high voltage FLASH memory An improved EEPROM that is cheaper, faster and has a higher density (so can store more)

Important Memory Attributes (1) Size The total storage capacity, usually measured in Megabits or Megabytes (increasingly, Gigabytes) Organisation The number of locations, and the number of bits per location. The size in bits is the product of these two (e.g. 1M×32 = 32 Megabits, or 4 Megabytes in 1 million 32-bit words)

Important Memory Attributes (2) Speed How quickly memory can respond to a read or write command; often called access time, and quoted in nanoseconds (e.g. 70 ns memory) Power consumption How much electricity it consumes, e.g. in milliwatts per megabyte. This is important for battery-powered computers. Faster memory consumes more power

Machine Code The commands the CPU understands

Machine Code Definition: A set of binary codes that are recognised and executed directly by a particular CPU

Machine Instructions An individual machine code is called a Machine Instruction –e.g. the machine instruction to add 1 to the value in accumulator A is (or 4C if you prefer it in hex) The set of all codes recognised by a particular CPU is known as its Instruction Set

Machine Instructions A typical machine instruction consists of an operation code (op-code), which specifies what operation the CPU is to do, plus a number of arguments, which specify what data the CPU is to operate on –e.g. the machine instruction to add 2 to the value in accumulator A is

What do/can machine instructions do? We can group the instructions according to function. The groups given here are generally applicable to most instruction sets (i.e. they apply to the machine code for most types of processor)

Instructions: data transfer From where, to where? load (e.g. from memory to a register) store (e.g. from a register to memory) move (e.g. from register to register) …

Instructions: computations The arithmetic and logical operations, normally carried out by the ALU What might this include? add subtract increment invert bits … and more

Instructions: flow control A computer program is not a lot of use without loops, functions, etc. We need to have machine codes to control the flow of execution through a machine code program Branching, jumping to subroutines, returning from subroutines

Instructions: others You’ll come across other types of instruction as the course proceeds The bulk, however, fall into the three categories already mentioned: Data transfer Computations Flow control

The Programmer’s Model This is the way a programmer needs to view the CPU As a programmer, you don’t necessarily need to know about what we’ve learnt before regarding the internals but you need to know what’s sometimes termed the Programmer’s Model of the CPU

LC-3 Microcomputer Programmer’s Model

LC-3 Overview: Memory and Registers Memory –address space: 2 16 locations (16-bit addresses) –addressability: 16 bits

Registers –temporary storage, accessed in a single machine cycle accessing memory generally takes longer than a single cycle –eight general-purpose registers: R0 - R7 each 16 bits wide how many bits to uniquely identify a register? –other registers not directly addressable, but used by (and affected by) instructions PC (program counter), condition codes

LC-3 Overview: Instruction Set Opcodes –15 opcodes –Operate instructions: ADD, AND, NOT –Data movement instructions: LD, LDI, LDR, LEA, ST, STR, STI –Control instructions: BR, JSR/JSRR, JMP, RTI, TRAP –some opcodes set/clear condition codes, based on result: N = negative, Z = zero, P = positive (> 0)

Instruction Set 2 Data Types –16-bit 2’s complement integer Addressing Modes –How is the location of an operand specified? –non-memory addresses: immediate, register –memory addresses: PC-relative, indirect, base+offset

Operate Instructions Only three operations: ADD, AND, NOT Source and destination operands are registers –These instructions do not reference memory. –ADD and AND can use “immediate” mode, where one operand is hard-wired into the instruction.

NOT (Register) Note: Src and Dst could be the same register.

ADD/AND register this zero means “register mode”

ADD/AND (Immediate) Note: Immediate field is sign-extended. this one means “immediate mode”

Using Operate Instructions With only ADD, AND, NOT… –How do we subtract? Subtract: R3 = R1 - R2 Take 2’s complement of R2, then add to R1. (1) R2 = NOT(R2) (2) R2 = R2 + 1 (3) R3 = R1 + R2 –How do we OR? OR: R3 = R1 OR R2 Use DeMorgan’s Law -- invert R1 and R2, AND, then invert result. (1) R1 = NOT(R1) (2) R2 = NOT(R2) (3) R3 = R1 AND R2 (4) R3 = NOT(R3) –How do we copy from one register to another? Register-to-register copy: R3 = R2 R3 = R2 + 0 (Add-immediate) –How do we initialize a register to zero?

Work arounds How do we OR? OR: R3 = R1 OR R2 Use DeMorgan’s Law -- invert R1 and R2, AND, then invert result. (1) R1 = NOT(R1) (2) R2 = NOT(R2) (3) R3 = R1 AND R2 (4) R3 = NOT(R3) How do we copy from one register to another? Register-to-register copy: R3 = R2 R3 = R2 + 0 (Add-immediate)

–How do we initialize a register to zero? Initialize to zero: R1 = 0 R1 = R1 AND 0 (And-immediate)

Data Movement Instructions Load -- read data from memory to register –LD: PC-relative mode –LDR: base+offset mode –LDI: indirect mode Store -- write data from register to memory –ST: PC-relative mode –STR: base+offset mode –STI: indirect mode Load effective address -- compute address, save in register –LEA: immediate mode –does not access memory

PC-Relative Addressing Mode Want to specify address directly in the instruction –But an address is 16 bits, and so is an instruction! –After subtracting 4 bits for opcode and 3 bits for register, we have 9 bits available for address. Solution: –Use the 9 bits as a signed offset from the current PC. 9 bits: Can form any address X, such that: Remember that PC is incremented as part of the FETCH phase; This is done before the EVALUATE ADDRESS stage.

LD (PC-Relative) The instruction loads data from PC plus the offset on bits 0-8

ST (PC-Relative) The instruction stores data from PC plus the offset on bits 0-8

Indirect Addressing Mode With PC-relative mode, can only address data within 256 words of the instruction. –What about the rest of memory? Solution #1: –Read address from memory location, then load/store to that address. First address is generated from PC and IR (just like PC-relative addressing), then content of that address is used as target for load/store.

LDI (Indirect) An address is formed as in the LD and ST forms. BUT the address pointed at contains the address of the operand. Therefore operand can be anywhere in memory. Likewise for STI to store anywhere in memory

Base + Offset Addressing Mode With PC-relative mode, can only address data within 256 words of the instruction. –What about the rest of memory? Solution #2: –Use a register to generate a full 16-bit address. 4 bits for opcode, 3 for src/dest register, 3 bits for base register -- remaining 6 bits are used as a signed offset. –Offset is sign-extended before adding to base register.

LDR (Base+Offset) This instruction use a register to hold base address (16bit) and adds offset to base. Loads contents pointed to by base + offset

STR (Base+Offset) This instruction use a register to hold base address (16bit) and adds offset to base. Stores contents of source in location pointed to by base + offset

Load Effective Address Computes address like PC-relative (PC plus signed offset) and stores the result into a register. Note: The address is stored in the register, not the contents of the memory location.

LEA (Immediate) Destination will contain address of PC +1 +/- offset.

Example AddressInstructionComments x30F R1  PC – 3 = x30F4 x30F R2  R = x3102 x30F M[PC - 5]  R2 M[x30F4]  x3102 x30F R2  0 x30FA R2  R2 + 5 = 5 x30FB M[R1+14]  R2 M[x3102]  5 x30FC R3  M[M[x30F4]] R3  M[x3102] R3  5 opcode

Control Instructions Used to alter the sequence of instructions (by changing the Program Counter) Conditional Branch –branch is taken if a specified condition is true signed offset is added to PC to yield new PC –else, the branch is not taken PC is not changed, points to the next sequential instruction Unconditional Branch (or Jump) –always changes the PC TRAP –changes PC to the address of an OS “service routine” –routine will return control to the next instruction (after TRAP)

Condition Codes LC-3 has three condition code registers: N -- negative Z -- zero P -- positive (greater than zero) Set by any instruction that writes a value to a register (ADD, AND, NOT, LD, LDR, LDI, LEA) Exactly one will be set at all times –Based on the last instruction that altered a register

Branch Instruction Branch specifies one or more condition codes. If the set bit is specified, the branch is taken. –PC-relative addressing: target address is made by adding signed offset (IR[8:0]) to current PC. –Note: PC has already been incremented by FETCH stage. –Note: Target must be within 256 words of BR instruction. If the branch is not taken, the next sequential instruction is executed.

BR (PC-Relative) What happens if bits [11:9] are all zero? All one?

Using Branch Instructions Compute sum of 12 integers. (Numbers start at location x3100. Program starts at location x3000. R1  x3100 R3  0 R2  12 R2=0? R4  M[R1] R3  R3+R4 R1  R1+1 R2  R2-1 NO YES

Sample Program AddressInstructionComments x R1  x3100 (PC+0xFF) x R3  0 x R2  0 x R2  12 x If Z, goto x300A (PC+5) x Load next value to R4 x Add to R3 x Increment R1 (pointer) X Decrement R2 (counter) x Goto x3004 (PC-6)

JMP (Register) Jump is an unconditional branch -- always taken. –Target address is the contents of a register. –Allows any target address.

TRAP Calls a service routine, identified by 8-bit “trap vector.” When routine is done, PC is set to the instruction following TRAP. vectorroutine x23input a character from the keyboard x21output a character to the monitor x25halt the program