Inside the Machine: An Illustrated Introduction to Microprocessors and Computer Architecture by jon stokes

unit, which, because it contains the instruction register and pro-
    gram counter, is the rudder of the CPU.
    The Branch Instruction as a Special Type of Load
    Recall that an instruction fetch is a special type of load that happens auto-
    matically for every instruction and that always takes the address in the program counter as its source and the instruction register as its destination. With that in mind, you might think of a branch instruction as a similar kind of load,
    but under the control of the programmer instead of the CPU. The branch
    instruction is a load that takes the address specified by #target as its source
    and the instruction register as its destination.
    Like a regular load, a branch instruction can take as its target an address
    stored in a register. In other words, branch instructions can use register-
    relative addressing just like regular load instructions. This capability is useful because it allows the computer to store blocks of code at arbitrary places in
    memory. The programmer doesn’t need to know the address where the
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    Chapter 2
    block of code will wind up before writing a branch instruction that jumps to
    that particular block; all he or she needs is a way to get to the memory
    location where the operating system, which is responsible for managing
    memory, has stored the starting address of the desired block of code.
    Consider Program 2-3, in which the programmer knows that the
    operating system has placed the address of the branch target in line 17 in
    register C. Upon reaching line 17, the computer jumps to the address stored
    in C by copying the contents of C into the instruction register.
    Line
    Code
    Comments
    16
    sub A, B, A
    Subtract the number in register A from the number in register B and
    store the result in A.
    17
    jumpz #C
    Check the PSW, and if the result of the previous instruction was
    zero, jump to the instruction at the address stored in C. If the result
    was nonzero, continue on to line 18.
    18
    add A, 15, A
    Add 15 to the number in A and store the result in A.
    Program 2-3: A conditional branch that uses an address stored in a register
    When a programmer uses register-relative addressing with a branch
    instruction, the operating system must load a certain register with the base
    address of the code segment in which the program resides. Like the data
    segment, the code segment is a contiguous block of memory cells, but its
    cells store instructions instead of data. So to jump to line 15 in the currently running program, assuming that the operating system has placed the base
    address of the code segment in C, the programmer could use the following
    instruction:
    Code
    Comments
    jump #(C + 30)
    Jump to the instruction located 30 bytes away from the start of the code
    segment. (Each instruction is 2 bytes in length, so this puts us at the 15
    instruction.)
    Branch Instructions and Labels
    In programs written for real-world architectures, branch targets don’t usually
    take the form of either immediate values or register-relative values. Rather,
    the programmer places a label on the line of code to which he or she wants to jump, and then puts that label in the branch’s target field. Program 2-4
    shows a portion of assembly language code that uses labels.
    sub A, B, A
    jumpz LBL1
    add A, 15, A
    store A, #(D + 16)
    LBL1: add A, B, B
    store B, #(D + 16)
    Program 2-4: Assembly language code that uses labels
    The Mechanics of Program Execution
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    In this example, if the contents of A and B are equal, the computer will
    jump to the instruction with the label LBL1 and begin executing there,
    skipping the instructions between the jump and the labeled add. Just as the
    absolute memory addresses used in load and store instructions are modified
    at load time to fit the location in memory of the program’s data segment,
    labels like LBL1 are changed at load time into memory addresses that reflect
    the location in memory of the program’s code segment.
    Excursus: Booting Up
    If