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[[Category:SPO600]][[Category:6502]][[Category:Assembly Language]][[Image:6502_Emulator_Screenshot.png|thumb|400px|right|A screen shot of the 6502 Emulator running [[6502_Emulator_Example_Code#Etch-a-Sketchtm_Style_Drawing|the drawing example code]].]]A simple web-based [[6502]] emulator is available at [http://6502.cdot.systems http://6502.cdot.systems]. This emulator is used in the [[SPO600]] course to teach 8-bit [[Assembly Language|assembly language]] before transitioning to more complex 64-bit x86_64 and [[AArch64]] assembly language.

The emulator has a text area for data entryentering and editing code, a small bit-mapped graphics screen (32x32 pixels), a character screen (80x25 characters), a debug area, a memory monitor, and a message area.

* Assemble - assembles the code in the text area, placing the resulting binary machine language code at $0600 and outputting any error messasges to the message area.

* Reset - changes resets the PC to $0600, clears state of the bitmap and character displays, and clears emulator (see the stack [[#Peripherals and zero pageMemory Map|Memory Map]] section below).* Hexdump - shows , in hexadecimal, the hexadecimal values stored in memory starting at $0600

The debugger will constantly show the stat value of the emulated 6502's registers. If you select the Debugger checkbox, you will be able to single-step through memory or jump to an address or label using the associated pushbuttons.

Selecting the Monitor checkbox will display the specified region of memory as code is executed. For example, specifying a start of $00 and a length of $100 will display the entire zero page. == Turning the Text Screen On/Off ==The checkbox labeled "Text Screen" can be used to hide the character display to free up more space for editing code. == Peripherals and Memory Map ==The entire 64K address space is available and is populated with RAM except for peripherals and a small ROM area. There are four peripherals available:* a one-byte pseudo-random number generator (''PRNG'') at '''$fe'''.* a single-key buffer at '''$ff ''' - if you write to this address, it will remain unchanged until a new keypress is received. Printable characters plus Return/Enter and Backspace are reported as ASCII codes; cursor keys are reported as $80=up, $81=right, $82=down, $83=left.* a 32x32 pixel bitmapped display at '''$0200-$05ff''', with one byte per pixel. The lowest four bits of each byte select one of 16 colours.* an 80x25 character display at '''$f000-$7cff''', with one byte per pixel. Printable ASCII characters will be displayed. If the high-order bit is set, the character will be shown in <span style="background: black; color: white;">&nbsp;reverse video&nbsp;</span>.* a read-only ROM chip is present at '''$fe00-$ffff'''; see below for details. The Reset button clears the zero page, bitmap display , and character display, and resets the stack pointer (SP=$ff), program counter (PC=$0600), status register (P=$30), and the general purpose registers (A=X=Y=$00). Code is assembled starting at $0600 (unless the code changes the location with a <code>*=''address''</code> directive). Memory following the program is reset to $00. A <code>BRK</code> instruction ($00) will cause the program to stop and return control to the debugger. For more details, press the Notes button in the emulator. == ROM Routines ==These routines are defined in ROM:* SCINIT $ff81 - Initialize and clear the character display* CHRIN $ffcf - Input one character from keyboard (returns A)* CHROUT $ffdw - Outputs one character (A) to the screen at the current cursor position. Screen will wrap/scroll appropriately. Printable characters and cursor codes ($80/$81/$82/$83 for up/right/left/down) as well as RETURN ($0d) are accepted. Printable ASCII codes with the high bit set will be printed in reverse video.* SCREEN $ffed - Returns the character screen size in the X and Y registers.* PLOT $fff0 - gets (CARRY=1) or sets (CARRY=0) the cursor position  If C=0: X,Y registers set the cursor position Y:A is returned as a pointer to the current screen position (i.e., Y is the high byte and A is the low byte of a pointer to the memory address of the current cursor position) If C=1: X,Y registers return the current cursor position A contains the character at the current cursor location Registers which are not used for input/output are not affected by the ROM routines. The ROM routines use $f0-fd in the entire zero page. To use the ROM routines, these define directives may be pasted into your code:  define SCINIT $ff81 ; initialize/clear screen define CHRIN $ffcf ; input character from keyboard define CHROUT $ffd2 ; output character to screen define SCREEN $ffed ; get screen size define PLOT $fff0 ; get/set cursor coordinates You can then access the routines by name using the <code>JSR</code> instruction:  jsr CHROUT {{Admon/tip|ROM Routine Names|The ROM routine names and locations pay homage to a famous 6502-based family of computers. Discovering which one is left as an exercise for the reader!}} == Example Code ==See the [[6502 Emulator Example Code]] page.

== Peripherals Source Code ==There The emulator's source code can be found at https://github.com/ctyler/6502js ... Pull Requests are four peripherals available: the bitmapped and character displays, a pseudo-random number generator, and a single-key keyboard interfacewelcome.