With the hardware ready, a computer requires software to make it more than an inactive collection of components. Microprocessors fetch instructions from program memory, each consisting of an opcode and, optionally, additional operands following the opcode. These opcodes are binary data that are easy for the microprocessor to decode, but they are not very readable by a person. To enable a programmer to more easily write software, an instruction representation called assembly language was developed. Assembly language is a low-level language that directly represents each binary opcode with a human-readable text mnemonic. For example, the mnemonic for an unconditional branch-to-subroutine instruction could be BSR. In contrast, a high-level language such as C++ or Java contains more complex logical expressions that may be automatically converted by a compiler to dozens of microprocessor instructions. Assembly language programs are assembled, rather than compiled, into opcodes by directly translating each mnemonic into its binary equivalent.
Assembly language also makes programming easier by enabling the usage of text labels in place
of hard-coded addresses. A subroutine can be named FOO, and when BSR FOO is encountered by the assembler, a suitable branch target address will be automatically calculated in place of the label FOO. Each type of assembler requires a slightly different format and syntax, but there are general assembly language conventions that enable a programmer to quickly adapt to specific implementations once the basics are understood. An assembly language program listing usually has three columns of text followed by an optional comment column as shown in Fig. 3.14. The first column is for labls that are placeholders for addresses to be resolved by the assembler. Instruction mnemonics are located in the second column. The third column is for instruction operands.
This listing uses the Motorola 6800 family’s assembly language format. Though developed in the
1970s, 68xx microprocessors are still used today in embedded applications such as automobiles and industrial automation. The first line of this listing is not an instruction, but an assembler directive that tells the assembler to locate the program at memory location $100. When assembled, the listing is converted into a memory dump that lists a range of memory addresses and their corresponding contents—opcodes and operands. Assembler directives are often indicated with a period prefix.
Assembly language also makes programming easier by enabling the usage of text labels in place
of hard-coded addresses. A subroutine can be named FOO, and when BSR FOO is encountered by the assembler, a suitable branch target address will be automatically calculated in place of the label FOO. Each type of assembler requires a slightly different format and syntax, but there are general assembly language conventions that enable a programmer to quickly adapt to specific implementations once the basics are understood. An assembly language program listing usually has three columns of text followed by an optional comment column as shown in Fig. 3.14. The first column is for labls that are placeholders for addresses to be resolved by the assembler. Instruction mnemonics are located in the second column. The third column is for instruction operands.
This listing uses the Motorola 6800 family’s assembly language format. Though developed in the
1970s, 68xx microprocessors are still used today in embedded applications such as automobiles and industrial automation. The first line of this listing is not an instruction, but an assembler directive that tells the assembler to locate the program at memory location $100. When assembled, the listing is converted into a memory dump that lists a range of memory addresses and their corresponding contents—opcodes and operands. Assembler directives are often indicated with a period prefix.
