程序代写代做代考 algorithm assembler assembly Assembly Language

Assembly Language

Chapter 9
TRAP Routines and
Subroutines

ECE 206 – Fall 2001 – G. Byrd

System Calls
Certain operations require specialized knowledge
and protection:
specific knowledge of I/O device registers
and the sequence of operations needed to use them
I/O resources shared among multiple users/programs;
a mistake could affect lots of other users!

Not every programmer knows (or wants to know)
this level of detail

Provide service routines or system calls
(part of operating system) to safely and conveniently
perform low-level, privileged operations

ECE 206 – Fall 2001 – G. Byrd

System Call
1. User program invokes system call.
2. Operating system code performs operation.
3. Returns control to user program.
In LC-3, this is done through the TRAP mechanism.

ECE 206 – Fall 2001 – G. Byrd

LC-3 TRAP Mechanism
1. A set of service routines.
part of operating system — routines start at arbitrary addresses
(convention is that system code is below x3000)
up to 256 routines

2. Table of starting addresses.
stored at x0000 through x00FF in memory
called System Control Block in some architectures

3. TRAP instruction.
used by program to transfer control to operating system
8-bit trap vector names one of the 256 service routines

4. A linkage back to the user program.
want execution to resume
immediately after the TRAP instruction

ECE 206 – Fall 2001 – G. Byrd

TRAP Instruction
Trap vector
identifies which system call to invoke
8-bit index into table of service routine addresses

in LC-3, this table is stored in memory at 0x0000 – 0x00FF
8-bit trap vector is zero-extended into 16-bit memory address

Where to go
lookup starting address from table; place in PC

How to get back
save address of next instruction (current PC) in R7

ECE 206 – Fall 2001 – G. Byrd

TRAP
NOTE: PC has already been incremented
during instruction fetch stage.

ECE 206 – Fall 2001 – G. Byrd

RET (JMP R7)
How do we transfer control back to
instruction following the TRAP?

We saved old PC in R7.
JMP R7 gets us back to the user program at the right spot.
LC-3 assembly language lets us use RET (return)
in place of “JMP R7”.

Must make sure that service routine does not
change R7, or we won’t know where to return.

ECE 206 – Fall 2001 – G. Byrd

TRAP Mechanism Operation
Lookup starting address.
Transfer to service routine.
Return (JMP R7).

ECE 206 – Fall 2001 – G. Byrd

Example: Using the TRAP Instruction
.ORIG x3000
LD R2, TERM ; Load negative ASCII ‘7’
LD R3, ASCII ; Load ASCII difference
AGAIN TRAP x23 ; input character
ADD R1, R2, R0 ; Test for terminate
BRz EXIT ; Exit if done
ADD R0, R0, R3 ; Change to lowercase
TRAP x21 ; Output to monitor…
BRnzp AGAIN ; … again and again…
TERM .FILL xFFC9 ; -‘7’
ASCII .FILL x0020 ; lowercase bit
EXIT TRAP x25 ; halt
.END

ECE 206 – Fall 2001 – G. Byrd

Example: Output Service Routine
.ORIG x0430 ; syscall address
ST R7, SaveR7 ; save R7 & R1
ST R1, SaveR1
; —– Write character
TryWrite LDI R1, CRTSR ; get status
BRzp TryWrite ; look for bit 15 on
WriteIt STI R0, CRTDR ; write char
; —– Return from TRAP
Return LD R1, SaveR1 ; restore R1 & R7
LD R7, SaveR7
RET ; back to user
CRTSR .FILL xF3FC
CRTDR .FILL xF3FF
SaveR1 .FILL 0
SaveR7 .FILL 0
.END
stored in table,
location x21

ECE 206 – Fall 2001 – G. Byrd

TRAP Routines and their Assembler Names

vector symbol routine
x20 GETC read a single character (no echo)
x21 OUT output a character to the monitor
x22 PUTS write a string to the console
x23 IN print prompt to console,
read and echo character from keyboard
x25 HALT halt the program

ECE 206 – Fall 2001 – G. Byrd

Saving and Restoring Registers
Must save the value of a register if:
Its value will be destroyed by service routine, and
We will need to use the value after that action.

Who saves?
caller of service routine?

knows what it needs later, but may not know what gets altered by called routine
called service routine?

knows what it alters, but does not know what will be needed later by calling routine

ECE 206 – Fall 2001 – G. Byrd

Example
LEA R3, Binary
LD R6, ASCII ; char->digit template LD R7, COUNT ; initialize to 10
AGAIN TRAP x23 ; Get char
ADD R0, R0, R6 ; convert to number
STR R0, R3, #0 ; store number
ADD R3, R3, #1 ; incr pointer
ADD R7, R7, -1 ; decr counter
BRp AGAIN ; more?
BRnzp NEXT
ASCII .FILL xFFD0
COUNT .FILL #10
Binary .BLKW #10
What’s wrong with this routine?
What happens to R7?

ECE 206 – Fall 2001 – G. Byrd

Saving and Restoring Registers
Called routine — “callee-save”
Before start, save any registers that will be altered
(unless altered value is desired by calling program!)
Before return, restore those same registers

Calling routine — “caller-save”
Save registers destroyed by own instructions or
by called routines (if known), if values needed later

save R7 before TRAP
save R0 before TRAP x23 (input character)
Or avoid using those registers altogether

Values are saved by storing them in memory.

ECE 206 – Fall 2001 – G. Byrd

Question
Can a service routine call another service routine?

If so, is there anything special the calling service routine
must do?

ECE 206 – Fall 2001 – G. Byrd

What about User Code?
Service routines provide three main functions:
1. Shield programmers from system-specific details.
2. Write frequently-used code just once.
3. Protect system resources from malicious/clumsy
programmers.

Are there any reasons to provide the same functions
for non-system (user) code?

ECE 206 – Fall 2001 – G. Byrd

Subroutines
A subroutine is a program fragment that:
lives in user space
performs a well-defined task
is invoked (called) by another user program
returns control to the calling program when finished

Like a service routine, but not part of the OS
not concerned with protecting hardware resources
no special privilege required

Reasons for subroutines:
reuse useful (and debugged!) code without having to
keep typing it in
divide task among multiple programmers
use vendor-supplied library of useful routines

ECE 206 – Fall 2001 – G. Byrd

JSR Instruction
Jumps to a location (like a branch but unconditional),
and saves current PC (addr of next instruction) in R7.
saving the return address is called “linking”
target address is PC-relative (PC + Sext(IR[10:0]))
bit 11 specifies addressing mode

if =1, PC-relative: target address = PC + Sext(IR[10:0])
if =0, register: target address = contents of register IR[8:6]

ECE 206 – Fall 2001 – G. Byrd

JSR
NOTE: PC has already been incremented
during instruction fetch stage.

ECE 206 – Fall 2001 – G. Byrd

JSRR Instruction
Just like JSR, except Register addressing mode.
target address is Base Register
bit 11 specifies addressing mode

What important feature does JSRR provide
that JSR does not?

ECE 206 – Fall 2001 – G. Byrd

JSRR
NOTE: PC has already been incremented
during instruction fetch stage.

ECE 206 – Fall 2001 – G. Byrd

Returning from a Subroutine
RET (JMP R7) gets us back to the calling routine.
just like TRAP

ECE 206 – Fall 2001 – G. Byrd

Example: Negate the value in R0
2sComp NOT R0, R0 ; flip bits
ADD R0, R0, #1 ; add one
RET ; return to caller

To call from a program (within 1024 instructions):

; need to compute R4 = R1 – R3
ADD R0, R3, #0 ; copy R3 to R0
JSR 2sComp ; negate
ADD R4, R1, R0 ; add to R1
…
Note: Caller should save R0 if we’ll need it later!

ECE 206 – Fall 2001 – G. Byrd

Passing Information to/from Subroutines
Arguments
A value passed in to a subroutine is called an argument.
This is a value needed by the subroutine to do its job.
Examples:

In 2sComp routine, R0 is the number to be negated
In OUT service routine, R0 is the character to be printed.
In PUTS routine, R0 is address of string to be printed.
Return Values
A value passed out of a subroutine is called a return value.
This is the value that you called the subroutine to compute.
Examples:

In 2sComp routine, negated value is returned in R0.
In GETC service routine, character read from the keyboard
is returned in R0.

ECE 206 – Fall 2001 – G. Byrd

Using Subroutines
In order to use a subroutine, a programmer must know:
its address (or at least a label that will be bound to its address)
its function (what does it do?)

NOTE: The programmer does not need to know
how the subroutine works, but
what changes are visible in the machine’s state
after the routine has run.
its arguments (where to pass data in, if any)
its return values (where to get computed data, if any)

ECE 206 – Fall 2001 – G. Byrd

Saving and Restore Registers
Since subroutines are just like service routines,
we also need to save and restore registers, if needed.

Generally use “callee-save” strategy,
except for return values.
Save anything that the subroutine will alter internally
that shouldn’t be visible when the subroutine returns.
It’s good practice to restore incoming arguments to
their original values (unless overwritten by return value).

Remember: You MUST save R7 if you call any other
subroutine or service routine (TRAP).
Otherwise, you won’t be able to return to caller.

ECE 206 – Fall 2001 – G. Byrd

Example
Write a subroutine FirstChar to:

find the first occurrence
of a particular character (in R0)
in a string (pointed to by R1);
return pointer to character or to end of string (NULL) in R2.

(2) Use FirstChar to write CountChar, which:
counts the number of occurrences
of a particular character (in R0)
in a string (pointed to by R1);
return count in R2.

Can write the second subroutine first,
without knowing the implementation of FirstChar!

ECE 206 – Fall 2001 – G. Byrd

CountChar Algorithm (using FirstChar)

save regs
call FirstChar
R3 <- M(R2) R3=0 R1 <- R2 + 1 restore regs return no yes save R7, since we’re using JSR ECE 206 - Fall 2001 - G. Byrd Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CountChar Implementation ; CountChar: subroutine to count occurrences of a char CountChar ST R3, CCR3 ; save registers ST R4, CCR4 ST R7, CCR7 ; JSR alters R7 ST R1, CCR1 ; save original string ptr AND R4, R4, #0 ; initialize count to zero CC1 JSR FirstChar ; find next occurrence (ptr in R2) LDR R3, R2, #0 ; see if char or null BRz CC2 ; if null, no more chars ADD R4, R4, #1 ; increment count ADD R1, R2, #1 ; point to next char in string BRnzp CC1 CC2 ADD R2, R4, #0 ; move return val (count) to R2 LD R3, CCR3 ; restore regs LD R4, CCR4 LD R1, CCR1 LD R7, CCR7 RET ; and return ECE 206 - Fall 2001 - G. Byrd Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FirstChar Algorithm save regs R2 <- R1 R3 <- M(R2) R3=0 R3=R0 R2 <- R2 + 1 restore regs return no no yes yes ECE 206 - Fall 2001 - G. Byrd Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FirstChar Implementation ; FirstChar: subroutine to find first occurrence of a char FirstChar ST R3, FCR3 ; save registers ST R4, FCR4 ; save original char NOT R4, R0 ; negate R0 for comparisons ADD R4, R4, #1 ADD R2, R1, #0 ; initialize ptr to beginning of string FC1 LDR R3, R2, #0 ; read character BRz FC2 ; if null, we’re done ADD R3, R3, R4 ; see if matches input char BRz FC2 ; if yes, we’re done ADD R2, R2, #1 ; increment pointer BRnzp FC1 FC2 LD R3, FCR3 ; restore registers LD R4, FCR4 ; RET ; and return ECE 206 - Fall 2001 - G. Byrd Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Library Routines Vendor may provide object files containing useful subroutines don’t want to provide source code -- intellectual property assembler/linker must support EXTERNAL symbols (or starting address of routine must be supplied to user) ... .EXTERNAL SQRT ... LD R2, SQAddr ; load SQRT addr JSRR R2 ... SQAddr .FILL SQRT Using JSRR, because we don’t know whether SQRT is within 1024 instructions. ECE 206 - Fall 2001 - G. Byrd

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