Passing Parameters using Stack Calling program pushes parameters on the stack one element at a time before calling subroutine. Subroutine Call (jsr, bsr)

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Presentation transcript:

Passing Parameters using Stack Calling program pushes parameters on the stack one element at a time before calling subroutine. Subroutine Call (jsr, bsr) then pushes the return address on stack. Subroutine should therefore provide an additional offset of four to access parameters on stack After returning from subroutine (rts), original parameters are still pushed Calling program must increment the stack pointer by the number of bytes the parameters occupy, in order to clean up the stack and bring the Stack Pointer (SP) to its original position.

; main program ; mainequ* … leastring, -(sp) ; move start address on stack clr.w-(sp) ; reserve space for result jsrconvert ; call subroutine move (sp)+, d0 ; result is in (sp), save it addi#4, sp ; clean up the stack … … ; end of code ; subroutine convert ; convertequ* movea.l6(sp), a2; first param is at 6(sp) ; second param is at 4(sp) …. move.wd1, 4(sp); assume result was in d1 rts; return ; ; data area ; stringds.b20 end SP  Return address4 bytes Space for result2 bytes String address4 bytes SP 

POWER Subroutine Example (Case 3) Parameter Passing by Value: Using The Stack - Main Program - MAINORG $400Main Program origin MOVEA.L#$07FFE,SPInitialize Stack Pointer MOVE.B B,D1 Put base number into D1 EXT.WD1Sign extend base to word length MOVE.W D1,-(SP) push base B onto the stack CLR.WD2Clear D2 before loading exponent MOVE.B E,D2 Put exponent number into D2 MOVE.W D2,-(SP) push exponent E onto the stack BSR POWER Call subroutine POWER MOVE.L (SP)+,D3 pop answer from stack resetting SP LEA A,A5 put address of answer into A5 MOVE.L D3,(A5)save answer MOVE#228,D7Done TRAP#14 ORG$600 B DC.B4Base number stored here E DC.B 2Exponent number stored here A DS.L1answer to be stored here

POWER Subroutine Example (Case 3) POWER Subroutine Example (Case 3) Parameter Passing by Value: Using The Stack Continued - Subroutine - ORG$800Subroutine POWER origin POWER MOVE.W 6(SP),D1copy base from stack to D1 CLR.WD2Clear D2 before loading exponent MOVE.B4(SP),D2copy exponent from to D2 MOVE.L #1,D3 initialize result in D3 to 1 LOOP MULS D1,D3 multiply result D3 with base D1 SUB #1,D2 decrement power in D2 by one BNE LOOPand repeat as long as power > 0 MOVE.LD3,4(SP)Push result onto the stack RTSDone, return to calling program

Effect on The Stack Word Just before calling Subroutine Current SP Base B Exponent E Initial SP Word Just before end of main Current SP = Initial SP Just after calling Subroutine Word Current SP Base B Exponent E Initial SP Return address 8 Word Just before return to main Current SP Answer Return address Initial SP 4 8 (Case 3)

POWER Subroutine Example (Case 4) POWER Subroutine Example (Case 4) Parameter Passing by Reference: Using The Stack - Main Program - MAINORG $400 Main Program origin MOVEA.L#$07FFE,SP Initialize Stack Pointer PEA B Push address of Base onto the stack PEA E Push address of Exponent onto the stack PEA A Push address of Answer onto the stack BSR POWER Call subroutine POWER LEA 12(SP),SP Stack clean-up: stack pointer reset MOVE#228,D7 Done TRAP#14 ORG$600 B DC.B4 Base number stored here E DC.B 2 Exponent number stored here A DS.L1 answer to be stored here

POWER Subroutine Example (Case 4) Parameter Passing by Reference: Using The Stack Continued - Subroutine - ORG$800Subroutine POWER origin POWER MOVEA.L12(SP),A1load Base address in A1 MOVEA.L 8(SP),A2load Exponent address in A2 MOVEA.L 4(SP),A3load Answer address address in A3 MOVE.B (A1),D1 Put base number into D1 EXT.WD1Sign extend base to word length CLR.WD2Clear D2 before loading exponent MOVE.B(A2),D2copy exponent from to D2 MOVE.L #1,D3 initialize result in D3 to 1 LOOP MULS D1,D3 multiply result D3 with base D1 SUB #1,D2 decrement power in D2 by one BNE LOOPand repeat as long as power > 0 MOVE.LD3,(A3)Save result in memory RTSDone, return to calling program

Effect on The Stack Word Just before calling Subroutine Current SP Initial SP Base Address Exponent Address Answer Address Word Just after calling Subroutine Current SP Initial SP Base Address Exponent Address Answer Address Return Address (Case 4) PEA B PEA E PEA A BSR POWER

Effect on The Stack Just after return to main Word Just after stack clean-up in Main Current SP = Initial SP Word Current SP Initial SP Base Address Exponent Address Answer Address (Case 4) LEA 12(SP),SP RTS

Saving/Restoring -Storage is in the order from a7 to a0, then d7 to d0 -Restore is in the opposite order, first d0 to d7, then a0 to a7

The MOVE Multiple: MOVEM Instruction This instruction saves or restores multiple registers. Useful in subroutines to save the values of registers not used to pass parameters. MOVEM has two forms: MOVEM register_list, MOVEM,register_list No effect on CCR. Example: Saving/restoring registers to from memory SUBR1MOVEM D0-D7/A0-A6,SAVEBLOCKSAVE D0-D7/A0-A6... MOVEM SAVEBLOCK,D0-D7/A0-A6Restore D0-D7/A0-A6 RTS Example: Saving/restoring registers using the stack (preferred method). SUBR1MOVEM D0-D7/A0-A6,-(SP)Push D0-D7/A0-A6 onto the stack... MOVEM (SP)+,D0-D7/A0-A6Restore D0-D7/A0-A6 from the stack RTS