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→Week 4 - Class II
=== Week 4 - Class II ===
* 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;"> 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" 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 alwaysis 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
