Download 16 - TAL 1

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 Rt,Rs,I Rt [Rs]+([I15]16||I15..0)
addiu Rt,Rs,I Rt [Rs]+([I15]16||I15..0)
andi Rt,Rs,I
Rt 016||([Rs]15..0 AND I15..0)
lui Rt,I
Rt I15..0||016
The MAL move instruction
move Rt, Rs
is equivalent to the TAL instruction
add Rt,$0,Rs
Each of the MAL shift instructions exists in
two forms in TAL
sll Rdest,Rsrc1,Src2
sllv Rdest,Rscr1,Rscr2
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 Rs,Rt
takes the contents of Rs and Rt as multiplicands
and places the least 32-bit result into LO and
the other 32-bit result into HI.
MAL
mul $8,$9,$10
TAL
mult $9,$10
mflo $8
move from LO
HI must be checked for overflow
Division in TAL
The TAL div instruction computes both the
integer quotient and remainder, leaving the
quotient in LO and the remainder in HI.
MAL
TAL
rem $8,$9,$10
div $9,$10
mfhi $8
div $8,$9,$10
div $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.
addu
multu
divu
addiu
subu
Branch Instructions
Branch instructions use a signed 216 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 217-1
addresses forward and 217 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
MAL
blt $11,$12,next
Subtraction may cause
an overflow, e.g. if $11
contains 4000000 and
$12 contains -4000000 then
the result is 8000000 which
has no representation
TAL
sub $10,$11,$12
bltz $10,next
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).
MAL
la R, label
TAL
lui R, highaddress
ori R, lowaddress
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 ored into
R using the ori instruction.
la $12, 0x00030248
lui $12, 0x0003
ori $12, 0x0248