True Assembly Language Part I

The Assembly Process The process of translating a MAL code (an assembly language program) into machine code (a sequence of bits) is called assembly. The program that performs the translation is called an assembler. The translation from assembly language to machine code is mechanical, repetitive, and tedious. Small errors can have disastrous results and therefore this job is better left to computers.

Sometimes, a set of instructions are repeated several times within a code. It is best to give a name to this repetitive code sequence. A macro defines a sequence of instructions by associating to it a keyword. A preprocessor can then be used to expand a macro into a series of instructions it represents. Many MAL instructions are identical to TAL. The first step in assembling a MAL program is to translate the code into TAL.

TAL - True Assembly Language MAL is not directly translatable into machine code because of several abstractions in the language. The TAL instruction set, however, defines the MIPS RISC architecture. TAL arithmetic and logical instructions generally require three-operand specifications but MAL permits two- or three-operand specifications.

Arithmetic and Logical Instructions TAL has both register and immediate addressing modes, although not all have immediate modes, e.g., there is no immediate sub instruction because it can be done by add addi R t,R s,I R t [R s ]+([I 15 ] 16 ||I ) addiu R t,R s,I R t [R s ]+([I 15 ] 16 ||I ) andi R t,R s,I R t 0 16 ||([R s ] AND I ) lui R t,I R t I ||0 16

The MAL move instruction move R t, R s is equivalent to the TAL instruction add R t,$0,R s Each of the MAL shift instructions exists in two forms in TAL sll R dest,R src1,Src2 sllv R dest,R scr1,R scr2 The first shift instruction is the same as MAL; the second one specifies a register from which the amount of shifts is obtained from. The suffix v indicate that the amount of shift is variable.

Multiplication in TAL MIPS RISC architecture contains two special registers, HI and LO, that are used to hold 64-bit results. mult R s,R t takes the contents of R s and R t as multiplicands and places the least 32-bit result into LO and the other 32-bit result into HI. mul $8,$9,$10 mult $9,$10 mflo $8 MALTAL HI must be checked for overflow move from LO

Division in TAL The TAL div instruction computes both the integer quotient and remainder, leaving the quotient in LO and the remainder in HI. rem $8,$9,$10div $9,$10 mfhi $8 MALTAL div $8,$9,$10div $9,$10 mflo $8

Unsigned arithmetic The MIPS RISC architecture provides a set of instructions that perform unsigned arithmetic via instructions that are suffixed u. These instructions never cause exceptions (error condition) even if overflow occurs. The responsibility is left to the programmer or compiler to assure that overflow never occurs when these instructions are used. adduaddiu multusubu divu

Branch Instructions Branch instructions use a signed 2 16 offset field, hence (because addresses must be multiples of 4, we can shift the 18 bit value right twice and just store a 16 bit value) can only jump addresses forward and 2 17 backward. Jump instructions can jump longer using a 26 -bit offset field. Not all MAL branch instructions are available in TAL. Only bltz, blez, bgez, blez, beq, bne, and b are available.

Subtle problems on branching blt $11,$12,next sub $10,$11,$12 bltz $10,next Subtraction may cause an overflow, e.g. if $11 contains and $12 contains then the result is which has no representation MAL TAL slt $10,$11,$12 bne $10,$0,next Sets $10 to 1 if the 2 ’s complement of the contents of $11 is less than the 2 ’s complement of the contents of $12, else it sets $10 to 0. The comparison does not cause overflow

Load Address Instruction TAL does not natively have the MAL la instruction (though la still works in TAL). The assembler determines the address bound to label and divides it into two halves. The upper half is loaded into R via lui and the lower half is or ed into R using the ori instruction. la R, labellui R, highaddress ori R, lowaddress MALTAL la $12, 0x lui $12, 0x0003 ori $12, 0x0248