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This web page examines the use of registers in assembly language. Specific examples of registers from various processors are used to illustrate the general nature of assembly language.
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Registers are fast memory, almost always connected to circuitry that allows various arithmetic, logical, control, and other manipulations, as well as possibly setting internal flags.
Most early computers had only one data register that could be used for arithmetic and logic instructions. Often there would be additional special purpose registers set aside either for temporary fast internal storage or assigned to logic circuits to implement certain instructions. Some early computers had one or two address registers that pointed to a memory location for memory accesses (a pair of address registers typically would act as source and destination pointers for memory operations). Computers soon had multiple data registers, address registers, and sometimes other special purpose registers. Some computers have general purpose registers that can be used for both data and address operations. Every digital computer using a von Neumann architecture has a register (called the program counter) that points to the next executable instruction. Many computers have additional control registers for implementing various control capabilities. Often some or all of the internal flags are combined into a flag or status register.
Accumulators are registers that can be used for arithmetic, logical, shift, rotate, or other similar operations. The first computers typically only had one accumulator. Many times there were related special purpose registers that contained the source data for an accumulator. Accumulators were replaced with data registers and general purpose registers. Accumulators reappeared in the first microprocessors.
Data registers are used for temporary scratch storage of data, as well as for data manipulations (arithmetic, logic, etc.). In some processors, all data registers act in the same manner, while in other processors different operations are performed are specific registers.
Address registers store the addresses of specific memory locations. Often many integer and logic operations can be performed on address registers directly (to allow for computation of addresses).
Sometimes the contents of address register(s) are combined with other special purpose registers to compute the actual physical address. This allows for the hardware implementation of dynamic memory pages, virtual memory, and protected memory.
The number of bits of an address register (possibly combined with information from other registers) limits the maximum amount of addressable memory. A 16-bit address register can address 64K of physical memory. A 24-bit address register can address address 16 MB of physical memory. A 32-bit address register can address 4 GB of physical memory. A 64-bit address register can address 1.8446744 x 1019 of physical memory. Addresses are always unsigned binary numbers. See number of bits.
General purpose registers can be used as either data or address registers.
Constant registers are special read-only registers that store a constant. Attempts to write to a constant register are illegal or ignored. In some RISC processors, constant registers are used to store commonly used values (such as zero, one, or negative one) — for example, a constant register containing zero can be used in register to register data moves, providing the equivalent of a clear instruction without adding one to the instruction set. Constant registers are also often used in floating point units to provide such value as pi or e with additional hidden bits for greater accuracy in computations.
Floating point registers are special registers set aside for floating point math.
Index registers are used to provide more flexibility in addressing modes, allowing the programmer to create a memory address by combining the contents of an address register with the contents of an index register (with displacements, increments, decrements, and other options). In some processors, there are specific index registers (or just one index register) that can only be used only for that purpose. In some processors, any data register, address register, or general register (or some combination of the three) can be used as an index register.
Base registers or segment registers are used to segment memory. Effective addresses are computed by adding the contents of the base or segment register to the rest of the effective address computation. In some processors, any register can serve as a base register. In some processors, there are specific base or segment registers (one or more) that can only be used for that purpose. In some processors with multiple base or segment registers, each base or segment register is used for different kinds of memory accesses (such as a segment register for data accesses and a different segment register for program accesses).
Control registers control some aspect of processor operation. The most universal control register is the program counter.
Almost every digital computer ever made uses a program counter. The program counter points to the memory location that stores the next executable instruction. Branching is implemented by making changes to the program counter. Some processor designs allow software to directly change the program counter, but usually software only indirectly changes the program counter (for example, a JUMP instruction will insert the operand into the program counter). An assembler has a location counter, which is an internal pointer to the address (first byte) of the next location in storage (for instructions, data areas, constants, etc.) while the source code is being converted into object code.
The VAX uses the 16th of 16 general purpose registers as the program counter (PC). Almost the entire instruction set can directly manipulate the program counter, allowing a very rich set of possible kinds of branching.
The program counter in System/360 and 370 machines is contained in bits 40-63 of the program status word (PSW), which is directly accessible by some instructions.
Processor flags store information about specific processor functions. The processor flags are usually kept in a flag register or a general status register. This can include result flags that record the results of certain kinds of testing, information about data that is moved, certain kinds of information about the results of compations or transformations, and information about some processor states. Closely related and often stored in the same processor word or status register (although often in a privileged portion) are control flags that control processor actions or processor states or the actions of certain instructions.
A few typical result flags (with processors that include them):
Some conditions are determined by combining multiple flags. For example, if a processor has a negative flag and a zero flag, the equivalent of a positive flag is the case of both the negative and zero flags both simultaneously being cleared.
A few typical control flags (with processors that include them):
Stack pointers are used to implement a processor stack in memory. In many processors, address registers can be used as generic data stack pointers and queue pointers. A specific stack pointer or address register may be hardwired for certain instructions. The most common use is to store return addresses, processor state information, and temporary variables for subroutines.
Some RISC processors include a special subroutine return pointer rather than using a stack in memory. The return address for subroutine calls is stored in this register rather than in memory. More than one level of subroutine calls requires storing and saving the contents of this register to and from memory.
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Created: February 14, 2001 (from asm.htm)
Last Updated: March 12, 2001
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