Assemblers & compilers translate for later execution by real hardware or by software interpreters. They are just like any other applications, and are best written in HLLs, especially those specific to the application area.
Simplifies the task of writing machine code programs
1) Lexical (word) analysis
2) Syntactic (sentence structure) analysis
3) Semantic (meaning) analysis
4) Code generation
(1) + (2) usually defined by a grammar (BNF) e.g.
instruction: mnemonic1 operand | mnemonic0 ; mnemonic1 : LDA | ADD | SUB | STO | JMP | JNE | JGE ; mnemonic0 : STP ;and then we would need to say that statements can have labels and comments, or just be numbers (for initialising variables) etc.
Makes it seem as if the computer can understand high-level language programs, as well as machine code programs.
/* division by repeated subtraction */ /* declarations */ Label again; Var ans=-1, a=99, b=6; /* commands */ again: ans=ans+1; a=a-b; If a>=0 JumpTo again; Stop;Same first 4 steps, but each is more complicated:
0 again: LDA ans 100B 1 ADD one 200C 2 STO ans 000B 3 LDA a 1009 4 SUB b 300A 5 STO a 0009 6 LDA a 1009 7 JGE again 5000 8 STP 7000 9 a: 99 0063 10 b: 6 0006 11 ans: -1 FFFF 12 one: 1 0001and:
Library: increases set of operations available to programmer,
allows access to facilities provided by system etc.
e.g. MU0 - provide multiplication and division
Compile these operations once, so users don't need to keep recompiling them.
Include list of operation names & addresses.
Linker: searches list(s) of names & addresses in libraries
to locate operations needed
Copies requested operations from the library into the user program.
In practice, there are lots of complications that mean libraries have to contain extra information and linkers have to do extra work:
e.g. when a library is being compiled, the compiler doesn't know where exactly it is going to be within each user program that needs it, so what addresses should it use? - the library includes extra information for the linker, which has to finalise (relocate) the addresses in the library.
e.g. what if one library needs to use the facilities provided by another library (e.g. a graphics library using sine and cosine functions from the maths library)? - the library includes extra information telling the linker what it needs, and the linker has to make extra searches to find them.