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The opcode for this instruction represents the Skipcond instruction. Bits ten
and eleven (read left to right, or bit eleven followed by bit ten) are 10, indicating
a value of 2. This implies a “skip if AC greater than or equal to 0.” If the value in
the AC is less than zero, this instruction is ignored and we simply go on to the
next instruction. If the value in the AC is greater than or equal to zero, this
instruction causes the PC to be incremented by 1, thus causing the instruction
immediately following this instruction in the program to be ignored (keep this in
mind as you read the following section on the instruction cycle).
These examples bring up an interesting point. We will be writing programs
using this limited instruction set. Would you rather write a program using the
commands Load, Add, and Halt, or their binary equivalents 0001, 0011, and
0111? Most people would rather use the instruction name, or mnemonic, for the
instruction, instead of the binary value for the instruction. Our binary instructions
are called machine instructions. The corresponding mnemonic instructions are
what we refer to as assembly language instructions. There is a one-to-one correspondence
between assembly language and machine instructions. When we type
in an assembly language program (i.e., using the instructions listed in Table 4.2),
we need an assembler to convert it to its binary equivalent. We discuss assemblers
in Section 4.5.
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