A small step up from the ISA of a machine is that ISA’s assembly language.
symbolic address symbolic and mnemonic name of opcodes and memory locations
The assembly translation program is called an assembler _and the translation process is called _assembly.
Assembly Language Program
Instructions
Opcodes and Operands
Opcode and operands are mandatory. A literal value must contain a symbol identifying the representation base of the number.
#for decimal,xfor hexadecimal,bfor binary.
Labels
Labels are symbolic names which are used to identify memory locations that are referred to explicity in the program.
In LC-3, a label consists of from 1 to 20 alphanumeric characters, starting with a letter of the alphabet.
reserved words the not-allwoed character strings
TWO conditions for explicity referring to a memory location, where we use LABEL :
- The location is the target of a branch instruction
- The location contains a value that is loaded or stored
:::
Comments
Comments are messages intended only human consumption. It’s important to make comments that provide additional insight and do not just restate the obvious.
Pseudo-Ops / Assembler Directives
like C/C++ that has
#pragma.
Pesudo-Op is a message from the assembly language programmer to the assembler to help the assemble in the assembly process.
Once the assemble handles the message, the pseudo-op is discarded.
:::danger
.ORIG
Where in the memory to place the LC-3 program. ::: :::danger
.FILL
To set aside the next location in the program and initialize it with the value of the operand. ::: :::danger
.BLKW
for a BLocK of Words
To set aside some number of sequential memory locations in the program and the content of memory locations will be clear to 0.
:::
:::danger
.STRINGZ
To initialize a sequence of N + 1 memory locations.
The argument is a sequence of N characters inside double quotation marks.
The last word containing
provides a convenient sentinel for processing the string of ASCII codes. Thus N + 1.
:::
:::danger
.END
To indicate that it has reached the end of the program and need not even look at anything after it. :::
Assembly Process
The First Pass : Creating the Symbol Table
It identify the actual binary addresses corresponding to the symbolic names ( or labels ). This set of correspondences is the symbol table.
location counter, LC A counter to keep track of the location assigned to each instruction. It’s initialized to the address specified in .ORIG.
The assembler examines each instruction in sequence and increments the LC once for each assembly language instrurction. And it constructs a symbol table like the following one :
The Second Pass : Generating the Machine Language Program
It translates the individual assembly language instructions into their corresponding machine language instructions with the help of the symbol table.
Other Concepts
The Executable Image
When a computer begins execution of a program, the entity being executed is called an executable image. The executable image is created from modules. Each modules translated separately into an object file. Each object file consists of instructions in the ISA of the computer being used, along with the associated data. The final step is to combine ( i.e. link ) all the object modules together into one executable image.
More than One Object File
The pseudo-op .EXTERNAL
It tells the assembler to create a symbol table entry for the label of .EXTERNAL and instead of assigning it an address, it would mark the symbol as belonging to another module.
At link time, all the modules are combined.
The .EXTERNAL pseudo-ops allows references by one module to symbolic locations in another module. The proper translations are resolved by the linker. :::
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