📄 chap4.asm
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; Chapter 4 6811 assembly language programs; Jonathan W. Valvano; This software accompanies the book,; Real Time Embedded Systems published by Brooks Cole;; Program 4.1. This subroutine is nonreentrant because of the read-modify-write access to a global. Second rmb 2 Temporary global variable* Input parameters: Reg X,Y contain 2 16 bit numbers* Output parameter: Reg X is returned with the averageAVE sty Second Save the second number in memory xgdx Reg D contains first number addd Second Reg D=First+Second lsrd (First+Second)/2 adcb #0 round up? adca #0 xgdx rts; Program 4.2. This subroutine is also nonreentrant because of the read-modify-write access to a global. Money rmb 2 bank balance implemented as a global* add $100 to the accountmore ldd Money where Money is a global variable addd #100 std Money Money=Money+100 rts; Program 4.4. This assembly subroutine is nonreentrant because of the write-read access to a global. temp rmb 2 temporary result implemented as a global* calculate RegX=RegX+2*RegDmac stx temp Save X so that it can be added lsld RegD=2*RegD addd temp RegD=RegX+2*RegD xgdx RegX=RegX+2*RegD rts ; Program 4.6. This assembly subroutine is nonreentrant because of the multi-step write access to a global. Info rmb 4 32-bit data implemented as a global* set the variable using RegX and RegYset stx Info Info is a 32 bit global variable sty Info+2 rts; Program 4.8. This assembly subroutine is reentrant because it does not write to any globals. * Input parameters: Reg X,Y contain 2 16 bit numbers* Output parameter: Reg X is returned with the averageAVE pshy Save the second number on the stack tsy Reg Y points the Second number xgdx Reg D contains first number addd 0,Y Reg D=First+Second lsrd (First+Second)/2 adcb #0 round up? adca #0 xgdx puly rts; Program 4.9. This assembly subroutine is nonreentrant because of the read-modify-write access to a global. Status rmb 1 0 means empty, -1 means it contains somethingMessage rmb 1 data to be communicated* Input parameter: Reg B contains an 8 bit message* Output parameter: Reg CC (C bit) is 1 for OK, 0 for busy errorSEND tst Status check if mailbox is empty bmi Busy full, can't store, so return with C=0 stab Message store dec Status signify it is now contains a message sec stored OK, so return with C=1Busy rts; Program 4.10. This assembly subroutine is reentrant because it disables interrupts during the critical section. Status rmb 1 0 means empty, -1 means it contains somethingMessage rmb 1 data to be communicated* Input parameter: Reg B contains an 8 bit message* Output parameter: Reg CC (C bit) is 1 for OK, 0 for busy errorSEND clc Initialize carry=0 tpa save current interrupt state psha sei disable interrupts during vulnerable window tst Status check if mailbox is empty bmi Busy full, so return with C=0 staa Message store dec Status signify it is now contains a message pula oraa #1 OK, so return with C=1 pshaBusy pula restore interrupt status tap rts; Program 4.12. This assembly subroutine can be used as part of a binary semaphore. * Global parameter: Semi4 is the memory location to test and set* If the location is zero, it will set it (make it -1)* and return Reg CC (Z bit) is 1 for OK* If the location is nonzero, it will return Reg CC (Z bit) = 0Semi4 fcb 0 Semaphore is initially freeTAS tst Semi4 check if already set bne Out busy, operation failed, so return with Z=0 dec Semi4 signify it is now busy bita #0 operation successful, so return with Z=1Out rts; Program 4.13. Code fragments showing the basic idea of a FIFO. GetPt rmb 2 Pointer to oldest data, next to be removed by GETPutPt rmb 2 Pointer to free memory, place to PUT next data* Reg A contains byte to store into the FIFOPutFifo ldx PutPt Reg X points to free place staa ,X Store data into FIFO inx Update pointer stx PutPt rts* Reg A returned with byte from FIFOGetFifo ldx GetPt Reg X points to oldest data ldaa ,X Read data from FIFO inx Update pointer stx GetPt rts; Program 4.15. Initialization of a two pointer FIFO. * Two pointer implementation of the FIFO FifoSize equ 10 Number of 8 bit data in the Fifo, less onePutPt rmb 2 Pointer of where to put next */GetPt rmb 2 Pointer of where to get next* FIFO is empty if PutPt=GetPt * FIFO is full if PutPt+1=GetPt (with wrap)Fifo rmb FifoSize The statically allocated fifo data ***********Initialize FIFO******************* No parametersInitFifo ldx #Fifo tpa sei make atomic, entering critical section stx PutPt stx GetPt Empty when PutPt=GetPt tap end critical section, restore CCR rts; Program 4.16. Assembly routine to put into a two pointer FIFO. ***********Put a byte into the FIFO******************* Input RegA contains 8 bit data to put* Output RegA is -1 if successful, 0 if data not storedPutFifo psha tpa tab pula pshb save old CCR sei make atomic, entering critical section ldx PutPt RegX is Temporary put pointer staa 0,x Try to put data into fifo inx cpx #Fifo+FifoSize bne PutNoWrap skip if no wrapping needed ldx #Fifo Wrap PutNoWrap clra assume it will fail cpx GetPt Full if now the same beq PutDone coma RegA=-1 means OK stx PutPtPutDone tab end critical section pula tpa restore CCR to previous value tba rts; Program 4.17. Assembly routine to get from a two pointer FIFO. ***********Get a byte from the FIFO******************* Input RegX points to place for 8 bit data from Get* Output RegA is -1 if successful, 0 if Fifo was empty when calledGetFifo tpa psha save old CCR sei make atomic, entering critical section clra assume it will fail ldy GetPt cpx PutPt Empty if initially the same beq GetDone coma RegA=-1 means OK ldab 0,y Data from FIFO stab 0,x Return by reference iny cpy #Fifo+FifoSize bne GetNoWrap skip if no wrapping needed ldy #Fifo Wrap GetNoWrap sty GetPtGetDone tab end critical section pula tpa restore CCR to previous value tba rts; Program 4.19. Initialization of a two pointer with counter FIFO. * Pointer,counter implementation of the FIFO FifoSize equ 10 Number of 8 bit data in the FifoPutPt rmb 2 Pointer of where to put next GetPt rmb 2 Pointer of where to get nextSize rmb 1* FIFO is empty if Size=0 * FIFO is full if Size=FifoSize Fifo rmb FifoSize The statically allocated fifo data ***********Initialize FIFO******************* No parametersInitFifo tpa sei make atomic, entering critical section ldx #Fifo stx PutPt stx GetPt Empty when Size == 0 clr Size tap end critical section rts; Program 4.20. Assembly routine to put into a two pointer with counter FIFO. ***********Put a byte into the FIFO******************* Input RegA contains 8 bit data to put* Output RegA is -1 if successful, 0 if data not storedPutFifo psha tpa tab pula pshb save old CCR sei make atomic, entering critical section ldab Size cmpb #FifoSize Full if Size==FifoSize bne PutNotFull clra bra PutDonePutNotFull incb stab Size Size++ ldx PutPt staa 0,x Put data into fifo inx cpx #Fifo+FifoSize bne PutNoWrap skip if no wrapping needed ldx #Fifo Wrap PutNoWrap ldaa #-1 success means OK stx PutPtPutDone tab end critical section pula tpa restore CCR to previous value tba rts; Program 4.21. Assembly routine to get from a two pointer with counter FIFO. ***********Get a byte from the FIFO******************* Input RegX points to place for 8 bit data from Get* Output RegA is -1 if successful, 0 if Fifo was empty when calledGetFifo tpa psha save old CCR sei make atomic, entering critical section clra assume it will fail tst Size beq GetDone dec Size ldy GetPt coma RegA=-1 means OK ldab 0,y Data from FIFO stab 0,x Return by reference iny cpy #Fifo+FifoSize bne GetNoWrap skip if no wrapping needed ldy #Fifo Wrap GetNoWrap sty GetPtGetDone tab end critical section pula tpa restore CCR to previous value tba rts; Program 4.23. Initialization of a two index with counter FIFO. * Index,counter implementation of the FIFO FifoSize equ 10 Number of 8 bit data in the Fifo PutI rmb 1 Index of where to put nextGetI rmb 1 Index of where to get nextSize rmb 1* FIFO is empty if Size=0 * FIFO is full if Size=FifoSize Fifo rmb FifoSize The statically allocated fifo data ***********Initialize FIFO******************* No parametersInitFifo tpa sei make atomic, entering critical section clr PutI clr GetI clr Size Empty when Size == 0 tap end critical section rts; Program 4.24. Assembly routine to put into a two index with counter FIFO. ***********Put a byte into the FIFO******************* Input RegA contains 8 bit data to put* Output RegA is -1 if successful, 0 if data not storedPutFifo psha⌨️ 快捷键说明
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