COMPUTER ORGANIZATION AND ASSEMBLY LANGUAGE Lecture 15 & 16 Stacks, Endianness Addressing Modes Course Instructor: Engr. Aisha Danish

Stacks  A stack is an ordered set of elements, only one of which can be accessed at a time.  The point of access is called the top of the stack  The number of elements in the stack, or length of the stack, is variable  The last element in the stack is the base of the stack  Items may only be added to or deleted from the top of the stack  A stack is also known as a pushdown list or a last-in-first- out (LIFO) list

Stacks

 A PUSH operation appends one new item to the top of the stack  A POP operation removes the top item from the stack  Binary operators, which require two operands (e.g., multiply, divide, add, subtract), use the top two stack items as operands, pop both items, and push the result back onto the stack  Unary operations, which require only one operand (e.g., logical NOT), use the item on the top of the stack

Stack Implementation  The stack is a useful structure to provide as part of a processor implementation  One use, is to manage procedure calls and returns  Stacks may also be useful to the programmer  The implementation of a stack depends in part on its potential uses  If it is desired to make stack operations available to the programmer, then the instruction set will include stack-oriented operations, including PUSH, POP, and operations that use the top one or two stack elements as operands  Because all of these operations refer to a unique location, namely the top of the stack, the address of the operand or operands is implicit and need not be included in the instruction. These are the zeroaddress instructions

Stack Implementation  Implementation of a stack requires some set of locations to be used to store the stack elements  A contiguous block of locations is reserved in main memory (or virtual memory) for the stack  Most of the time, the block is partially filled with stack elements and the remainder is available for stack growth  Three addresses are needed for proper operation, and these are often stored in processor registers: Stack pointer (SP) Contains the address of the top of the stack. If an item is appended to or deleted from the stack, the pointer is incremented or decremented to contain the address of the new top of the stack. Stack base Contains the address of the bottom location in the reserved block. If an attempt is made to POP when the stack is empty, an error is reported. Stack limit Contains the address of the other end of the reserved block. If an attempt is made to PUSH when the block is fully utilized for the stack, an error is reported.

Stack Implementation

Stack implementations have two key attributes: Ascending/descending  Ascending stack grows in the direction of ascending addresses, starting from a low address and progressing to a higher address  SP is incremented when items are pushed  Decremented when items are pulled  A descending stack grows in the direction of descending addresses, starting from a high address and progressing to a lower one  Most machines implement descending stacks as a default

Stack Implementation Full/empty  A misleading terminology, because is does not refer to whether the stack is completely full or completely empty  Rather, the SP can either point to the top item in the stack (full method), or the next free space on the stack (an empty method)  For the full method, when the stack is completely full, the SP points to the upper limit of the stack. For the empty method, when the stack is completely empty, the SP points to the base of the stack The ARM architecture allows the system programmer to specify the use of ascending or descending, empty or full stack operations The x86 architecture uses a descending/empty convention

LITTLE-, BIG-,AND BI-ENDIAN Byte Ordering  The concept of endianness was first discussed in the literature by Cohen [COHE81]  Suppose we have the 32-bit hexadecimal value  It is stored in a 32-bit word in byte-addressable memory at byte location 184  The value consists of 4 bytes, with the least significant byte containing the value 78  The most significant byte containing the value 12.  There are two obvious ways to store this value:

LITTLE-, BIG-,AND BI-ENDIAN Big-Endian  The mapping that stores the most significant byte in the lowest numerical byte address; is known as big endian and is equivalent to the left-to-right order of writing in Western culture languages Little-Endian  The mapping that stores the least significant byte in the lowest numerical byte address;is known as little endian

LITTLE-, BIG-,AND BI-ENDIAN  Some machines, such as the Intel 80x86, x86, are little-endian machines  IBM System 370/390, the Motorola 6800, Sun SPARC, and most RISC machines, are big endian  The PowerPC is a bi-endian processor that supports both big-endian and little-endian modes

Addressing Modes  The address field or fields in a typical instruction format are relatively small  We want to reference a large range of locations in main memory or, for some systems, virtual memory  To achieve this objective, a variety of addressing techniques has been employed

Addressing Modes  Immediate  Direct  Indirect  Register  Register Indirect  Displacement (Indexed)  Stack

Notations (X) = contents of memory location X or register X EA = actual (effective) address of the location containing the referenced operand R = contents of an address field in the instruction that refers to a register A = contents of an address field in the instruction

Immediate Addressing  Operand is part of instruction  Operand = address field  e.g. ADD 5  Add 5 to contents of accumulator  5 is operand  No memory reference to fetch data  Fast  Limited range

Immediate Addressing Diagram OperandOpcode Instruction

Direct Addressing  Address field contains address of operand  Effective address (EA) = address field (A)  e.g. ADD A  Add contents of cell A to accumulator  Look in memory at address A for operand  Single memory reference to access data  No additional calculations to work out effective address  Limited address space

Direct Addressing Diagram Address AOpcode Instruction Memory Operand

Indirect Addressing (1)  Memory cell pointed to by address field contains the address of (pointer to) the operand  EA = (A)  Look in A, find address (A) and look there for operand  e.g. ADD (A)  Add contents of cell pointed to by contents of A to accumulator

Indirect Addressing (2)  Large address space  May be nested, multilevel, cascaded  e.g. EA = (((A))) Draw the diagram yourself  Multiple memory accesses to find operand  Hence slower

Indirect Addressing Diagram Address AOpcode Instruction Memory Operand Pointer to operand

Register Addressing (1)  Operand is held in register named in address filed  EA = R  Limited number of registers  Very small address field needed  Shorter instructions  Faster instruction fetch

Register Addressing (2)  No memory access  Very fast execution  Very limited address space  Multiple registers helps performance  Requires good assembly programming or compiler writing

Register Addressing Diagram Register Address ROpcode Instruction Registers Operand

Register Indirect Addressing  EA = (R)  Operand is in memory cell pointed to by contents of register R  Large address space  One fewer memory access than indirect addressing

Register Indirect Addressing Diagram Register Address ROpcode Instruction Memory Operand Pointer to Operand Registers

Displacement Addressing  A very powerful mode of addressing combines the capabilities of direct addressing and register indirect addressing  Displacement addressing requires that the instruction have two address fields, at least one of which is explicit  The value contained in one address field (value = A) is used directly  The other address field refers to a register whose contents are added to A to produce the effective address. Three of the most common uses of displacement addressing:  Relative addressing  Base-register addressing  Indexing

Displacement Addressing  EA = A + (R)  Address field hold two values  A = base value  R = register that holds displacement  or vice versa

Displacement Addressing Diagram Register ROpcode Instruction Memory Operand Pointer to Operand Registers Address A +

Relative Addressing  For relative addressing, also called PC-relative addressing, the implicitly referenced register is the program counter (PC)  The next instruction address is added to the address field to produce the EA.  Typically, the address field is treated as a twos complement number for this operation.  Thus, the effective address is a displacement relative to the address of the instruction.  If most memory references are relatively near to the instruction being executed, then the use of relative addressing saves address bits in the instruction.

Relative Addressing  A version of displacement addressing  R = Program counter, PC  EA = A + (PC)  i.e. get operand from A cells from current location pointed to by PC

Base-Register Addressing  The referenced register contains a main memory address, and the address field contains a displacement from that address  The register reference may be explicit or implicit.

Base-Register Addressing  A holds displacement  R holds pointer to base address  R may be explicit or implicit  convenient means of implementing segmentation  e.g. segment registers in 80x86

Indexing  The address field references a main memory address, and the referenced register contains a positive displacement from that address  Just the opposite of the interpretation for base-register addressing  Because the address field is considered to be a memory address in indexing, it generally contains more bits than an address field in a comparable base-register instruction  The method of calculating the EA is the same for both base-register addressing and indexing  In both cases the register reference is sometimes explicit and sometimes implicit (for different processor types).

Indexing: Example  Indexing provides an efficient mechanism for performing iterative operations  Consider a list of numbers stored starting at location A. Suppose that we would like to add 1 to each element on the list. We need to fetch each value, add 1 to it, and store it back. The sequence of effective addresses that we need is A,A+1, A+2..., up to the last location on the list  With indexing, this is easily done. The value A is stored in the instruction’s address field, and the index register is initialized to 0. After each operation, the index register is incremented by 1.

Indexing  Autoindexing: Because index registers are commonly used for such iterative tasks, it is typical that there is a need to increment or decrement the index register after each reference to it. Because this is a common operation, some systems will automatically do this as part of the same instruction cycle. This is known as autoindexing.  Autoindexing using increment can be depicted as follows EA = A + (R) (R) <--- (R) + 1

Indexing  A = base  R = displacement  EA = A + R  Good for accessing arrays  EA = A + R  R++

Indexing  Postindex If indexing is performed after the indirection, it is termed postindexing: EA = (A) + (R)  Preindex If indexing is performed before the indirection, it is termed preindexing: EA = (A+(R))  (Draw the diagrams)

Stack Addressing  The stack mode of addressing is a form of implied addressing. The machine instructions need not include a memory reference but implicitly operate on the top of the stack.  Operand is on top of stack  e.g.  ADDPop top two items from stack and add

x86 Addressing Modes  Virtual or effective address is offset into segment  Starting address plus offset gives linear address  This goes through page translation if paging enabled  12 addressing modes available  Immediate  Register operand  Displacement  Base  Base with displacement  Scaled index with displacement  Base with index and displacement  Base scaled index with displacement  Relative

x86 Addressing Modes  x86 address translation mechanism produces an address, called a virtual or effective address, that is an offset into a segment  The sum of the starting address of the segment and the effective address produces a linear address  The x86 is equipped with a variety of addressing modes intended to allow the efficient execution of high-level languages

x86 Addressing Modes

ARM Addressing Modes  Typically, a RISC machine, unlike a CISC machine, uses a simple and relatively straightforward set of addressing modes  The ARM architecture departs somewhat from this tradition by providing a relatively rich set of addressing modes  These modes are most conveniently classified with respect to the type of instruction

ARM Addressing Modes  LOAD/STORE ADDRESSING Load and store instructions are the only instructions that reference memory. This is always done indirectly through a base register plus offset. There are three alternatives with respect to indexing:  Offset  Preindex  Postindex

ARM Addressing Modes

 DATA PROCESSING INSTRUCTION ADDRESSING Data processing instructions use either register addressing of a mixture of register and immediate addressing  BRANCH INSTRUCTIONS The only form of addressing for branch instructions is immediate addressing

ARM Addressing Modes  LOAD/STORE MULTIPLE ADDRESSING Load Multiple instructions load a subset (possibly all) of the general-purpose registers from memory Store Multiple instructions store a subset (possibly all) of the general-purpose registers to memory The list of registers for the load or store is specified in a 16-bit field in the instruction Each bit corresponding to one of the 16 registers Load and Store Multiple addressing modes produce a sequential range of memory addresses