Changes

* ==== Strings and System Routines====** The [[6502 Emulator|6502 emulator]] has a 80x25 character display mapped starting at location '''$f000'''. Writing to a byte to screen memory will cause that character to be displayed at the corresponding location on the screen, if the character is printable. If the high bit is set, the character will be displayed in <span style="background:black;color:white;">&nbsp;reverse video </span>. For example, storing the ASCII code for "A" (which is 65 or $41) into memory location $f000 will display the letter "A" as the first character on the screen; ORing the value with 128 ($80) yields a value of 193 or $d1, and storing that value into $f000 will display <span style="background:black;color:white;">A</span> as the first character on the screen.** A "ROM chip" with screen control routines is mapped into the emulator at the end of the memory space (at the time of writing, the current version of the ROM exists in pages $fe and $ff). Details of the available ROM routines can be viewed using the "Notes" button in the emulator or on the [[6502_Emulator#ROM_Routines|emulator page]] on this wiki.** Strings in assembler are stored as sequences of bytes. As is usually the case in assembler, memory management is left to the programmer. You can terminate strings with null bytes (C-style), which are easy to detect one some CPUs (e.g., <code>lda</code> followed by <code>bne / beq</code> on a 6502), or you can use character counts to track string lengths.

* ==== Building Code====** C code is built with the C compiler, typically called <code>cc</code> (which is usually an alias for a specific C compiler, such as <code>gcc</code>, <code>clang</code>, or <code>bcc</code>).** The C compiler runs through four five steps, often by calling separate executables:**# Preprocessing - performed by the C Preprocessor (<code>cpp</code>), this step handles directives such as <code>#include</code>, <code>#define</code>, and <code>#ifdef</code> to build produce a single source code text file, with cross-references to the original input files so that error messages can be displayed correctly (e.g., an error in an included file can be correctly reported by filename and line number).**# Compilation - the C source code is converted to assembler, going through one or more intermedie representations (IR) such as [https://gcc.gnu.org/onlinedocs/gccint/GENERIC.html GENERIC] or [https://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html GIMPLE], or [https://llvm.org/docs/LangRef.html LLVM IR]. The program used for this step is often called <code>cc1</code>.**# Optimization - various optimization passes are performed at different stages of processingthrough multiple passes, but centered on IR at the compilation step. Sometimes, the work of a previous pass is undone by a later pass: for example, a complex loop may be converted into a series of simpler loops by an early pass, in the hope that optimizations can be applied to one or more of the simpler loops; the loops may later be recombined to single loop if no optimizations are found that are applicable to the simplified loops.**# Assembly - converts the assembly language code emitted by the compilation stage into binary object code.**# Linking - connects code to functions (aka methods or procedures) which were compiled in other ''compilation units'' (they may be pre-compiled libraries available on the system, or they may be other pieces of the same code base which are compiled in separate steps). Linking may be static, where libraries are imported into the binary executable file of the output program, or linking may be dynamic, where additional information is added to the binary executable file so that a run-time linker can load and connect libraries at runtime.