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Recitation Material of Engineering an Assembler

Learn how to load, recognize, and assemble instructions using an assembler in this comprehensive engineering material.

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Recitation Material of Engineering an Assembler

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  1. Recitation Material of Engineering an Assembler June 11, 2013

  2. Step 1. Program Load • Make sure where the code will be loaded, so that the corresponding address is correct. • In modern assembler, address is determined when source file is loaded ~~~

  3. 0x00400018 0x00000003 [00400024] 23bdfff8  addi $sp, $sp, -8 [00400028] afbf0000  sw $ra, 0($sp) [0040002c] 34020001  ori $v0, $0, 1; 16: li $v0, 1 # 0! = 1 [00400030] 10800006  beq $a0, $0, 24 [zero-0x00400030][00400034] afa40004  sw $a0, 4($sp) [0040003c] 0c100009  jal 0x00400024 [factorial] [00400040] 8fa40004  lw $a0, 4($sp) [00400044] 70821002  mul $v0, $a0, $v0 [00400048] 8fbf0000  lw $ra, 0($sp) [0040004c] 23bd0008  addi $sp, $sp, 8 [00400050] 03e00008  jr $ra [00400054] 23bdfffc  addi $sp, $sp, -4     [main: Entry]    [00400058] afbf0000  sw $ra, 0($sp) [0040005c] 3c011001  lui $at, 4097 [prompt]; 39: la $a0, prompt [00400060] 34240000  ori $a0, $at, 0 [prompt] [00400064] 34020004  ori $v0, $0, 4; 40: li $v0, 4 [00400068] 0000000c  syscall [0040006c] 34020005  ori $v0, $0, 5; 43: li $v0, 5 [00400070] 0000000c  syscall [00400074] 00022021  addu $a0, $0, $v0 [00400078] 0c100009  jal 0x00400024 [factorial]; [0040007c] 00022021  addu $a0, $0, $v0 [00400080] 34020001  ori $a0, $0, 1; 50: li $v0, 1 [00400084] 0000000c  syscall [00400088] 3c011001  lui $at, 4097 [endl]; 53: la $a0, endl [0040008c] 3424001f  ori $a0, $1, 31 [endl] [00400090] 34020004  ori $v0, $0, 4; 54: li $v0, 4 [00400094] 0000000c  syscall [00400098] 34020000  ori $at, $0, 0; 57: li $v0, 0 # Return zero. [0040009c] 8fbf0000  lw $ra, 0($sp) [004000a0] 23bd0004  addi $sp, $sp, 4[004000a4] 03e00008  jr $ra; 61: jr $ra 0x0040007c 0x00000002 0x00400040 0x00000001 0x00400040 0x00400040 0x7ffffdfc $sp step 33

  4. Step 2. Instruction Table • Take addi $sp, $sp, -8 for example Addi is rt imm rs 8 00100000 00000000 00000000 000000000 Which is 0x 20 00 00 00

  5. Step 3. Register Table (optional) • For example, the table could be an array, which is in format: $sp, $a0, $v0, $ra, $t1, $t2, $t3 • Then you can use a lookup-table function, the return value is offset or index of the corresponding value of the register name(Notice the value is in array format).

  6. Lookup –Register Function # the lookup_vartable_func need to be called multiple times lookup_vartable_func: add $sp, $sp -4 # push v1 first sw $v1, 0($sp) add $sp, $sp, -4 # push a0 second sw $a0, 0($sp) add $sp, $sp, -4 # push a1 third sw $a1, 0($sp) add $sp, $sp, -4 # push t7 fourth sw $t7, 0($sp) add $sp, $sp, -4 # push t8 fifth sw $t8, 0($sp) add $sp, $sp, -4 # push s3 sixth sw $s3, 0($sp) add $sp, $sp, -4 # push s3 sw $s4, 0($sp) # now we begin to compare variable with parameter buffer # this_prepare_variable: # set up reference key for variable table li $v0, 0 li $a0, 0 # offset pointer in parameter buffer li $a1, 0 # set matching flag = 0 li $t8, 0x2c # t8 = comma or , li $t7, 0x28 # t7 = left parenthesis or ( lb $v1, parameter_buffer($a0) li $s4, 0 lb $s3, value_buffer($s4)

  7. Lookup –Register Function (Cont. ) this_finish_all_keys: add $v0, $v0, 1 beqz $a1, this_return_match_var li $v0, 0 this_return_match_var: lw $s4, 0($sp) add $sp, $sp, 4 lw $s3, 0($sp) add $sp, $sp, 4 lw $t8, 0($sp) add $sp, $sp, 4 lw $t7, 0($sp) add $sp, $sp, 4 lw $a1, 0($sp) add $sp, $sp, 4 lw $a0, 0($sp) add $sp, $sp, 4 lw $v1, 0($sp) jr $ra this_not_set_cur_flag: add $a0, $a0, 1 add $s4, $s4, 1 lb $s3, value_buffer($s4) lb $v1, parameter_buffer($a0) j this_lookup_variable_table this_finish_compare_one: add $v0, $v0, 1 beqz $a1, this_return_match_var li $s4, 0 add $a0, $a0, 1 lb $s3, value_buffer($s4) lb $v1, parameter_buffer($a0) # reset a1 to be 0 to prepare next comparison li $a1, 0 j this_lookup_variable_table

  8. Step 4. Recognize Token and Assemble Instructions • Shift bit according to recognized opcode say value of $rs is loaded using: lw $t3, register_value_table($s0) sll $t3, $t3, 21 • Say your addiopcode is loaded into $t1 now we assemble them, say: add $t2, $t1, $t3

  9. Step 5. Output instructions • Simply create an output buffer and sequentially copy each instruction first • Copy 0xffffffff ffffffff to separate .text segment and .data segment • Copy all data into .data segment

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