当我RTFM时，必须在汇编中实现AFAIK中断处理。
eret指令用于在异常前地址恢复执行。
这个原因也可以推广到x86，无论芯片是什么，你都不能编写没有汇编指令的操作系统？这是一个实际结果还是在理论上得到证实？汇编语言和导致这种差异的其他语言之间是否存在一些主要区别？我推测汇编语言没有所谓的BNF，这是真的吗？所以汇编语言没有所谓的无上下文语法，汇编语言没有用yacc，bison，flex，lex实现，但更像是根据硬件芯片？
我正在使用的低级代码看起来像这样，我想知道为什么它不能在C（或Java等）中完成，因为这些结果似乎与2图灵完整实现可以解决同样的问题相矛盾，所以如果语言A是完整的，语言B也是完整的，那么A能够解决的任何问题也可以用语言B解决。
如果你能教我这些概念，我想我自己可以学习个别指示，但我不知道如何回答有关汇编编程的问题，例如被问到为什么我用汇编代替C，那么为什么C而不是OOP等。
################################################################
#
# Definitions for important devices and addresses in this system.
#
# Uart_0 at 0x860
.equ de2_uart_0_base,0x860
# Timer_1 at 0x920, interrupt index 10 (mask 2^10 = 0x400)
.equ de2_timer_1_base,0x920
.equ de2_timer_1_intmask,0x400
# Timeout value for 0,1 ms tick-count interval (CHANGED in every version)
.equ de2_timer_1_timeout_value,4999
# Required tick count per time-slice, meaning
# the number of timer-interrupts before a thread-switch is performed
.equ oslab_ticks_per_timeslice,100
# Interrupt address at 0x800020
.equ de2_nios2_interrupt_address,0x800020
#
# End of device-address definitions
#
################################################################
################################################################
#
# Definition of variables for keeping system time etcetera.
#
.data
.align 2
.global oslab_internal_globaltime
oslab_internal_globaltime: .word 0
# Definition of variable for remembering the number of
# timer-interrupts since the last thread-switch
.data
.align 2
.global oslab_internal_tickcount
oslab_internal_tickcount: .word 0
# Definition of system (interrupt) stack, sp, and gp
.data
.align 2
oslab_internal_gp: .word 0
oslab_internal_sp: .word 0
oslab_system_stack: .fill 256,1,0
oslab_system_stacktop:
# Definition of the end-of-timeslice message.
oslab_internal_yield_message:
.asciz "\n#### Thread yielded after using %d tick%c."
#
# End of system-time variable definitions.
#
################################################################
################################################################
#
# Interrupt handling code.
#
# Stub for interrupt handler
.text
oslab_internal_stub:
movia et,oslab_exception_handler
jmp et
# The interrupt handler
oslab_exception_handler:
# Check source of exception, following the procedure
# described in the Nios II Processor Reference Handbook.
rdctl et,estatus # Check ESTATUS
andi et,et,1 # Test EPIE
beq et,r0,oslab_exception_was_not_an_interrupt
rdctl et,ipending # Check IPENDING
beq et,r0,oslab_exception_was_not_an_interrupt
# If control comes here, we have established that the
# exception was caused by an interrupt.
# Subtract 4 from ea, so that the interrupted instruction
# will be re-run when we return.
subi ea,ea,4
# Check the source of the interrupt.
# Possible source No. 1: Timer_1 (currently the only source).
rdctl et,ipending
andi et,et,de2_timer_1_intmask
bne et,r0,oslab_timer_1_interrupt
# If control comes here, we have an interrupt from an unknown source.
# This condition is IGNORED in this version of OSLAB.
eret
oslab_exception_was_not_an_interrupt:
# Test if the interrupted instruction was a TRAP
subi sp,sp,4 # PUSH r8 (instruction 1)
stw r8,0(sp) # PUSH r8 (instruction 2)
movia r8,0x003b683a # binary code for TRAP
ldw et,-4(ea) # Load interrupted instruction
cmpeq et,et,r8 # Compare to binary code for TRAP
# Result from comparison is now in et.
ldw r8,0(sp) # POP r8 (instruction 1)
addi sp,sp,4 # POP r8 (instruction 2)
# Use the comparison result in et as branch condition.
# The value in et will also be used later, to tell if the
# exception was a trap or an interrupt.
bne et,r0,oslab_trap_handler
# If control comes here, we have an exception which was not a TRAP.
# This should not normally happen.
# However, someone writing programs for the OSLAB micro-operating system
# could perhaps use unimplemented instructions. To catch unimplemented
# instructions, we insert a BREAK instruction here. This will stop execution
# unless the program is run through the debugger.
break 0
eret
oslab_timer_1_interrupt:
# Acknowledge the timer_1 interrupt.
movia et,de2_timer_1_base
stw r0,0(et)
# Save contents of R8, to get a free register for
# temporary values.
subi sp,sp,4
stw r8,0(sp) # PUSH r8
# Increase system clock.
movia r8,oslab_internal_globaltime
ldw et,0(r8)
addi et,et,1
stw et,0(r8)
# Increase tick counter.
movia r8,oslab_internal_tickcount
ldw et,0(r8)
addi et,et,1
stw et,0(r8)
# Restore original contents of R8.
ldw r8,0(sp) # POP r8
addi sp,sp,4
# Check value of tick counter,
# against the required number of ticks per time-slice.
# Note: oslab_ticks_per_timeslice is an assembler constant,
# and not a variable. Hence, no load/store-instructions here.
subi et,et,oslab_ticks_per_timeslice
# If the result from the subtraction is zero (or perhaps positive),
# then it is time to switch threads.
bge et,r0,oslab_time_to_switch
# If we fall-through here, then we have had one of those many
# timer interrupts on which we should not switch threads.
# Return to caller.
eret
oslab_time_to_switch:
# This code will now fall-through into the TRAP handler
# which performs a context switch.
#
# We will print out a message for each timer interrupt.
# To be able to tell that we had a timer interrupt, and not
# a TRAP, we set et to zero.
movi et,0
oslab_trap_handler:
# Save registers r1 through r23, plus fp, gp, ra and ea
.set noat # R1 is used here.
subi sp,sp,108 # Make room for all registers.
stw r1, 4(sp) # R1 is saved in slot 1, not slot 0.
stw r2, 8(sp)
stw r3,12(sp)
stw r4,16(sp)
stw r5,20(sp)
stw r6,24(sp)
stw r7,28(sp)
stw r8,32(sp)
stw r9,36(sp)
stw r10,40(sp)
stw r11,44(sp)
stw r12,48(sp)
stw r13,52(sp)
stw r14,56(sp)
stw r15,60(sp)
stw r16,64(sp)
stw r17,68(sp)
stw r18,72(sp)
stw r19,76(sp)
stw r20,80(sp)
stw r21,84(sp)
stw r22,88(sp)
stw r23,92(sp)
stw r26,96(sp)
stw r28,100(sp)
stw r31,104(sp)
stw ea,0(sp) # Special case, saved in slot 0.
mov r4,sp # Copy stack pointer to param1 register
movia sp,oslab_system_stacktop # Use system stack instead
# Test et to see if this was a timeout event or a TRAP.
beq et,r0,oslab_not_a_trap
# If this was a trap event, we fall through here.
# Our simplified printf is used to print a message,
# saying that the previous thread yielded parts of its time-slice.
################################################################
#
# The following code prints a nice message. Nothing more.
# This code saves and restores all registers it uses.
# You can safely ignore the following code, up to
# (but NOT including) the label oslab_not_a_trap.
#
subi sp,sp,4 # Contents of r4 must be preserved.
stw r4,0(sp) # PUSH r4.
movia r4,oslab_internal_yield_message
movia r5,oslab_internal_tickcount
ldw r5,0(r5)
movi r6,0 # Gold-plating: check if 1 tick or several ticks.
subi et,r5,1 # Do not print the s if only 1 tick.
beq et,r0,oslab_no_plural_ticks
movi r6,'s' # If 0 ticks, or 2 or more ticks, print the s.
oslab_no_plural_ticks:
call printf
ldw r4,0(sp) # POP r4
addi sp,sp,4
#
# This comment marks the end of the code for printing a nice message.
# Now comes other code, which is potentially much more interesting.
#
################################################################
# Move on to thread-switch code.
oslab_not_a_trap:
# Clear tick counter, since we are going to switch threads.
movia et,oslab_internal_tickcount
stw r0,0(et)
# Now it is time to execute the thread-switch code.
# We use the more general callr, rather than call.
movia et,oslab_internal_threadswitch
callr et # Call thread switch routine written in C
mov sp,r2 # Copy return value to stack pointer
# Yes, the system stack pointer is lost,
# but who cares? We will not need it any more.
# restore registers
ldw r1, 4(sp)
ldw r2, 8(sp)
ldw r3,12(sp)
ldw r4,16(sp)
ldw r5,20(sp)
ldw r6,24(sp)
ldw r7,28(sp)
ldw r8,32(sp)
ldw r9,36(sp)
ldw r10,40(sp)
ldw r11,44(sp)
ldw r12,48(sp)
ldw r13,52(sp)
ldw r14,56(sp)
ldw r15,60(sp)
ldw r16,64(sp)
ldw r17,68(sp)
ldw r18,72(sp)
ldw r19,76(sp)
ldw r20,80(sp)
ldw r21,84(sp)
ldw r22,88(sp)
ldw r23,92(sp)
ldw r26,96(sp)
ldw r28,100(sp)
ldw r31,104(sp)
ldw ea,0(sp) # Special case
addi sp,sp,108
eret # Return from exception
#
# End of exception handling code.
#
################################################################
################################################################
#
# Startup code.
#
# When the system is started, Altera-supplied code initializes the
# Nios II CPU and cache memories, and then calls alt_main.
#
.global alt_main
alt_main:
wrctl status,r0 # Disable interrupts.
wrctl ienable,r0 # Clear all bits in IENABLE.
# Now copy the stub.
movia r8,oslab_internal_stub
movia r9,de2_nios2_interrupt_address
ldw r10,0(r8)
stw r10,0(r9)
ldw r10,4(r8)
stw r10,4(r9)
ldw r10,8(r8)
stw r10,8(r9)
# Initialize timer_1.
movia r8,de2_timer_1_base
movia r9,de2_timer_1_timeout_value
srli r10,r9,16
stw r10,12(r8) # Write periodh
andi r10,r9,0xffff
stw r10,8(r8) # Write periodl
movi r10,7 # Continuous, interrupt on timeout, and start
stw r10,4(r8)
# Initialize CPU for interrupts from timer_1.
movi r10,de2_timer_1_intmask
wrctl ienable,r10
movi r10,1
wrctl status,r10
# Call to main. Do not jump, main is a subroutine,
# and may execute a ret instruction.
subi sp,sp,4
stw ra,0(sp) # PUSH r31
movia r8,main
callr r8
ldw ra,0(sp) # POP r31
addi sp,sp,4
# If main returns, we will return directly to the routine
# that called us (that called alt_main).
ret
#
# End of startup code.
#
################################################################
################################################################
#
# Helper functions for initialization and thread handling.
#
.text
.align 2
.global oslab_internal_get_gp
oslab_internal_get_gp:
mov r2,gp
ret
.global oslab_begin_critical_region
oslab_begin_critical_region:
wrctl status,r0
ret
.global oslab_end_critical_region
oslab_end_critical_region:
movi r8,1
wrctl status,r8
ret
.global oslab_get_internal_globaltime
oslab_get_internal_globaltime:
movia r2,oslab_internal_globaltime
ldw r2,0(r2)
ret
.global oslab_get_internal_tickcount
oslab_get_internal_tickcount:
movia r2,oslab_internal_tickcount
ldw r2,0(r2)
ret
.global oslab_yield
oslab_yield:
trap
ret
#
# End of helper functions.
#
################################################################
#
# ********************************************************
# *** You don't have to study the code below this line ***
# ********************************************************
#
################################################################
#
# A simplified printf() replacement.
# Implements the following conversions: %c, %d, %s and %x.
# No format-width specifications are allowed,
# for example "%08x" is not implemented.
# Up to four arguments are accepted, i.e. the format string
# and three more. Any extra arguments are silently ignored.
#
# The printf() replacement relies on routines
# out_char_uart_0, out_hex_uart_0,
# out_number_uart_0 and out_string_uart_0
# in file oslab_lowlevel_c.c
#
# We need the macros PUSH and POP - definitions follow.
# PUSH reg - push a single register on the stack
.macro PUSH reg
subi sp,sp,4 # reserve space on stack
stw \reg,0(sp) # store register
.endm
# POP reg - pop a single register from the stack
.macro POP reg
ldw \reg,0(sp) # fetch top of stack contents
addi sp,sp,4 # return previously reserved space
.endm
.text
.global printf
printf:
PUSH ra # PUSH return address register r31.
PUSH r16 # R16 will point into format string.
PUSH r17 # R17 will contain the argument number.
PUSH r18 # R18 will contain a copy of r5.
PUSH r19 # R19 will contain a copy of r6.
PUSH r20 # R20 will contain a copy of r7.
mov r16,r4 # Get format string argument
movi r17,0 # Clear argument number.
mov r18,r5 # Copy r5 to safe place.
mov r19,r6 # Copy r6 to safe place.
mov r20,r7 # Copy r7 to safe place.
asm_printf_loop:
ldb r4,0(r16) # Get a byte of format string.
addi r16,r16,1 # Point to next byte
# End of format string is marked by a zero-byte.
beq r4,r0,asm_printf_end
cmpeqi r9,r4,92 # Check for backslash escape.
bne r9,r0,asm_printf_backslash
cmpeqi r9,r4,'%' # Check for percent-sign escape.
bne r9,r0,asm_printf_percentsign
asm_printf_doprint:
# No escapes present, just print the character.
movia r8,out_char_uart_0
callr r8
br asm_printf_loop
asm_printf_backslash:
# Preload address to out_char_uart_0 into r8.
movia r8,out_char_uart_0
ldb r4,0(r16) # Get byte after backslash
addi r16,r16,1 # Increase byte count.
# Having a backslash at the end of the format string
# is illegal, but must not crash our printf code.
beq r4,r0,asm_printf_end
cmpeqi r9,r4,'n' # Newline
beq r9,r0,asm_printf_backslash_not_newline
movi r4,10 # Newline
callr r8
br asm_printf_loop
asm_printf_backslash_not_newline:
cmpeqi r9,r4,'r' # Return
beq r9,r0,asm_printf_backslash_not_return
movi r4,13 # Return
callr r8
br asm_printf_loop
asm_printf_backslash_not_return:
# Unknown character after backslash - ignore.
br asm_printf_loop
asm_printf_percentsign:
addi r17,r17,1 # Increase argument count.
cmpgei r8,r17,4 # Check against maximum argument count.
# If maximum argument count exceeded, print format string.
bne r8,r0,asm_printf_doprint
cmpeqi r9,r17,1 # Is argument number equal to 1?
beq r9,r0,asm_printf_not_r5 # beq jumps if cmpeqi false
mov r4,r18 # If yes, get argument from saved copy of r5.
br asm_printf_do_conversion
asm_printf_not_r5:
cmpeqi r9,r17,2 # Is argument number equal to 2?
beq r9,r0,asm_printf_not_r6 # beq jumps if cmpeqi false
mov r4,r19 # If yes, get argument from saved copy of r6.
br asm_printf_do_conversion
asm_printf_not_r6:
cmpeqi r9,r17,3 # Is argument number equal to 3?
beq r9,r0,asm_printf_not_r7 # beq jumps if cmpeqi false
mov r4,r20 # If yes, get argument from saved copy of r7.
br asm_printf_do_conversion
asm_printf_not_r7:
# This should not be possible.
# If this strange error happens, print format string.
br asm_printf_doprint
asm_printf_do_conversion:
ldb r8,0(r16) # Get byte after percent-sign.
addi r16,r16,1 # Increase byte count.
cmpeqi r9,r8,'x' # Check for %x (hexadecimal).
beq r9,r0,asm_printf_not_x
movia r8,out_hex_uart_0
callr r8
br asm_printf_loop
asm_printf_not_x:
cmpeqi r9,r8,'d' # Check for %d (decimal).
beq r9,r0,asm_printf_not_d
movia r8,out_number_uart_0
callr r8
br asm_printf_loop
asm_printf_not_d:
cmpeqi r9,r8,'c' # Check for %c (character).
beq r9,r0,asm_printf_not_c
# Print character argument.
br asm_printf_doprint
asm_printf_not_c:
cmpeqi r9,r8,'s' # Check for %s (string).
beq r9,r0,asm_printf_not_s
movia r8,out_string_uart_0
callr r8
br asm_printf_loop
asm_printf_not_s:
asm_printf_unknown:
# We do not know what to do with other formats.
# Print the format string text.
movi r4,'%'
movia r8,out_char_uart_0
callr r8
ldb r4,-1(r16)
br asm_printf_doprint
asm_printf_end:
POP r20
POP r19
POP r18
POP r17
POP r16
POP ra
ret
#
# End of simplified printf() replacement code.
#
################################################################
.end
答案 0 :(得分：2)
程序集通常与芯片的ISA具有1：1的关系。汇编指令通常具有以下形式：
<opcode> <lhs>,<rhs>
这可以在尽可能低的水平上工作，因此程序员可以处理各个处理器的功能。
程序集仍然需要一个由处理器运行的汇编程序。 assembler将符号程序集转换为处理器理解的二进制表示。
这类似于编译器的工作方式。在最高级别，编译器通常将源代码转换为abstract syntax tree，并从那里生成代码，可以是汇编代码，使用其他语言代码或机器代码。
理论上，编译器可以为任何语言编写，以生成任意机器代码。这意味着，如果编译器知道如何解释更高级别的代码，理论上也可以处理程序集可以访问的任何指令（例如中断）。
与this answer一样，大多数操作系统都会为您处理中断，并使用信号将其抽象出来。但是没有什么能阻止你自己处理它们，例如，在C ++中Arduino makes this possible。但问题是操作系统不允许任何程序访问中断，特别是因为某些中断需要特权CPU mode。
另外，没有什么可以阻止你使用OO语言（或函数等）来实现像内核这样的低级语言，但语言越复杂，生成高效的机器代码就越困难，以及何时你正在构建其他软件将运行的软件，它需要尽可能快。您将无法使用垃圾收集等功能（这在许多OO语言中很常见），因为没有任何功能可以在您之后进行清理。
OO并没有什么固有的东西使它变得缓慢，并且组装的任何固有内容都不会让它变得更快。当您确切知道代码正在做什么时，为处理器编写代码会更容易。
当你用C语言写作时，你只比汇编高出一级。它的高级别足以为您提供函数，结构和变量等概念，但足够低，您可以对生成的代码做出合理的假设。事实上，反汇编程序是一种优化C代码的好方法。尝试使用像Java这样的东西！
答案 1 :(得分：1)
编程语言倾向于避免或最好地平均掉它们运行的处理器的功能。大多数系统都需要操作（从中断返回是一个很好的例子）但不能直接用高级语言实现。有一些解决方案，一个是编译器特定的指令，在函数声明中放入一些单词的中断将告诉一些编译器这个函数是特殊的，需要包装，使它可以是一个中断处理程序，这是通常不同于正常功能的框架。
因为永远不会有通用的汇编语言/指令集，所以你不能创建通用的高级语言功能来匹配硬件，它没有意义，只需要在几行汇编中解决这些问题就是如此微不足道，即使有一个通用指令集，我怀疑高级语言会浪费任何努力来处理这些功能。
即使使用C，也需要一定程度的自举，可能设置堆栈指针，将.data放在正确的位置，将.bss归零等等。有趣的是，Cortex-M处理器内核已经将一些东西硬编码到硬件中您可以创建几乎没有程序集的应用程序，向量表是C代码以外的所有要求，并且通常该向量表只是asm指令，但可能在该表中没有asm指令。如果编辑器的调用约定发生变化，那么突然之间硬件不匹配就会出现问题，所以这将会持续多长时间。由于这些是微控制器而不是更高级别的系统，因此解决方案无法转换。