Indirect branch
Encyclopedia
An indirect branch is a type of program control instruction
present in some machine language instruction set
s. Rather than specifying the address
of the next instruction to execute
, as in a direct branch
, the argument
specifies where the address is located. Thus an example could be to 'jump indirect on the r1 register
', which would mean that the next instruction to be executed would be at the address whose value is in register r1. The address to be jumped to is not known until the instruction is executed. Indirect branches can also depend on the value of a memory location.
An indirect branch can be useful to make a conditional branch, especially a multiway branch
. For instance, based on program input
, a value could be looked up in a jump table of pointers to code
for handling the various cases implied by the data value. The data value could be added to the address of the table, with the result stored in a register. An indirect jump could then be made based on the value of that register, efficiently dispatching program control to the code appropriate to the input.
In a similar manner, subroutine
call instructions can be indirect, with the address of the subroutine to be called specified in a register or memory location.
Control flow
In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated....
present in some machine language instruction set
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
s. Rather than specifying the address
Memory address
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
of the next instruction to execute
Execution (computers)
Execution in computer and software engineering is the process by which a computer or a virtual machine carries out the instructions of a computer program. The instructions in the program trigger sequences of simple actions on the executing machine...
, as in a direct branch
Branch (computer science)
A branch is sequence of code in a computer program which is conditionally executed depending on whether the flow of control is altered or not . The term can be used when referring to programs in high level languages as well as program written in machine code or assembly language...
, the argument
Parameter (computer science)
In computer programming, a parameter is a special kind of variable, used in a subroutine to refer to one of the pieces of data provided as input to the subroutine. These pieces of data are called arguments...
specifies where the address is located. Thus an example could be to 'jump indirect on the r1 register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
', which would mean that the next instruction to be executed would be at the address whose value is in register r1. The address to be jumped to is not known until the instruction is executed. Indirect branches can also depend on the value of a memory location.
An indirect branch can be useful to make a conditional branch, especially a multiway branch
Multiway branch
Multiway branch is a computer science term used to describe the change to a program's control flow based upon a value matching a selected criteria. It is a form of conditional statement...
. For instance, based on program input
Information
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
, a value could be looked up in a jump table of pointers to code
Machine code
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...
for handling the various cases implied by the data value. The data value could be added to the address of the table, with the result stored in a register. An indirect jump could then be made based on the value of that register, efficiently dispatching program control to the code appropriate to the input.
In a similar manner, subroutine
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
call instructions can be indirect, with the address of the subroutine to be called specified in a register or memory location.
Examples
Z80: jp (hl)
SPARCSPARCSPARC is a RISC instruction set architecture developed by Sun Microsystems and introduced in mid-1987....
: jmpl %o7
MIPSMIPS architectureMIPS is a reduced instruction set computer instruction set architecture developed by MIPS Technologies . The early MIPS architectures were 32-bit, and later versions were 64-bit...
: jmpl %ra
X86: jmp *%eax
ARMARM architectureARM is a 32-bit reduced instruction set computer instruction set architecture developed by ARM Holdings. It was named the Advanced RISC Machine, and before that, the Acorn RISC Machine. The ARM architecture is the most widely used 32-bit ISA in numbers produced...
: mov pc, r2
IA64: br.ret.sptk.few rp
6502: jmp ($0DEA)