Transcript Assembly Language

Assembly Language
part 2
Program example 2
go past data – minimizes offset
BR 0x0008
calculations (contrast with data after code)
.ASCII "#?"
“constant” declarations
.ASCII "\n"
“variable” declaration
.BLOCK 2
CHARO 0x0003, d
output prompt
CHARO 0x0004, d
read number
DECI 0x0006, d
CHARO 0x0005, d output newline character
DECO 0x0006, d
output number
STOP
.END
Data declaration & storage
• The previous example included two
different types of data declaration
instructions:
– the .ASCII pseudo-op is used to allocate a
contiguous set of bytes large enough to hold
the specified data; as with Java & C++, the
backslash character (\) is used as the escape
character for control codes, like newline
– the .BLOCK pseudo-op allocates the specified
number of bytes and initializes their values to 0
Data declaration & storage
• Two other pseudo-ops provide data
storage:
– the .WORD instruction allocates two bytes,
suitable for storage of integers
– the .BYTE instruction allocates one byte,
suitable for storage of characters
– like .ASCII (and unlike .BLOCK), both of these
instructions allow the programmer to specify
initial values, as shown on next slide
Initialization examples
• .WORD 7
; allocates 2 bytes, with
; decimal value 7
• .BYTE 0x2B ; allocate 1 byte, with hex
; value 2B (‘+’)
I/O instructions
• The DECI and DECO instructions considerably
ease the process of reading and writing
numbers
• Each one deals with word-size data, and
represent instructions not available in the
underlying machine language – thus they are
part of the set of unimplemented op codes
• The actual I/O is performed by the operating
system; the instructions generate program
interrupts that allow the OS to temporarily take
over to provide a service to the program
I/O instructions
• The CHARI and CHARO instructions are
simply assembly language versions of the
machine language input and output
instructions:
– read or write a byte of data
– data source (for output) and destination (for
input) are memory (not registers)
I/O instructions
• STRO is yet another example of an
unimplemented op code
• Outputs a string of data
– String can be predefined with the .ASCII
pseudo-op
– Predefined string must be terminated with a
null character: “\x00”
Arranging instructions and data
• In the first program example (see Monday’s
notes), as with all of the machine language
examples, instructions were placed first, ended
with a STOP code, and data followed
• Problems with this approach:
– requires address calculations based on the number of
instructions (which may not be known as you’re
writing a particular instruction)
– addresses may have to be adjusted if even minor
changes are made to the program
Putting the data first
• An easy solution to the problems
described on the previous slide was
illustrated by the program example; the
solution is twofold:
– declare the data first
– place an unconditional branch instruction at
the beginning of the program, pointing to the
first instruction after the data
– the following example provides another
illustration
Program example 3
br 0x0020 ; bypass data
.block 4 ; space for 2 ints
.ascii "Enter a number: \x00"
.ascii " + \x00"
.ascii " = \x00"
stro 0x0007,d ; prompt
deci 0x0003,d ; get 1st number
stro 0x0007,d ; prompt
deci 0x0005,d ; get 2nd number
deco 0x0003,d ; output 1st number
stro 0x0018,d ; output ascii string " + "
deco 0x0005,d ; output 2nd number
stro 0x001c,d ; output string " = "
lda 0x0003,d ; put the first # in A
adda 0x0005,d ; add 2nd # to first
sta 0x0003,d ; store sum
deco 0x0003,d ; output sum
stop
.end
Program example 4: using labels
br code
pirate: .ASCII "Arrr!\x00"
code: stro pirate ,d
STOP
.END