Writing an Assembly-language program MIPS assembly language using MARS: MIPS Assembler and Runtime Simulator CS-2710 Dr. Mark L. Hornick 1

CS-2710 Dr. Mark L. Hornick 2 The MARS Assembler …is a modern, “single pass” Assembler Converts mnemonics to machine instructions (opcodes + operands) Ex: add $t0, $s1, $s2 is assembled to 0x Case on instructions is irrelevant; alternately: ADD $t0, $s1, $s2 Case on registers matters: $T0 is not permitted Reports syntactical errors Ex: add $t0, $s1 (e.g. too few operands) Can generate files containing machine code in a format that can be downloaded to real hardware

CS-2710 Dr. Mark L. Hornick 3 MARS also implements a Debugger/Simulator Simulates execution of the compiled program Start, stop, single-step Backwards step Breakpoints Can view contents of memory, registers Tracks cycles required to execute And lots more – see the help documentation

CS-2710 Dr. Mark L. Hornick 4 Number representation in MIPS assembly language All values are decimal unless notated otherwise addi $t0, $s1, 100 # decimal 100 (default) addi $t0, $s1, 0144 # octal 100 (note leading 0) addi $t0, $s1, 0x64 # hex 100 (note leading 0x) Radix prefixes 0 – octal 0x, 0X – hexadecimal 0x002a is equivalent to 0x2A

CS-2710 Dr. Mark L. Hornick 5 Labels can be used in place of actual addresses Every input line can be preceded by a label an alphanumeric string terminated by a colon (:) Labels are used as targets for jump and branch instructions The assembler automatically figures out what address to assign to a label

CS-2710 Dr. Mark L. Hornick 6 Every input line can be preceded by a label Example: Start: addi $t0, $s1, 100 # t0 = s Here: j Here# repeat forever Labels are alphanumeric strings terminated by a colon(:) Labels are given the value of the location in memory where the following instruction will be placed Labels can be used as jump and branch targets in program instructions The label Start: is assigned the value of the location of the addi instruction (0x ); Here: is assigned 0x c, the location of the j instruction The assembler automatically figures out what address to assign to a label

CS-2710 Dr. Mark L. Hornick 7 Directives are used within an assembly language program to control certain aspects of the operation of the Assembler Directives begin with a period (.) Directive often take operands, but not always Ex:.text Ex:.byte 1,2,3,4 Case does not matter.TEXT is equivalent Directives are not instructions – they are not translated to machine instructions They only “direct” the Assembler how to interpret subsequent assembly language instructions All directives are documented in the online help of MARS Directives may appear anywhere in an assembly program

CS-2710 Dr. Mark L. Hornick 8 Some assembler directives are used to define where to place things in memory:.text [loc] Alerts the Assembler that subsequent assembly statements are intended to generate instructions for the Text Segment of main memory ie, where executable machine code is placed Location set by MARS memory configuration settings If absent, a default value will be used by the Assembler.data [loc] Directs the Assembler where to begin placing subsequent data in memory Example:.data.byte 1,2,3,4 directs the subsequent data bytes 1,2,3,4,5 to be placed in the data segment of main memory