CS-280 Dr. Mark L. Hornick 1 Calling subroutines in assembly And using the Stack

CS-280 Dr. Mark L. Hornick 2 CS2851 Review: the Queue Queue: a traditional CS data structure Classical semantics (behavior) are: Elements can only be inserted at the back and removed from the front of the collection This is known as First-In, First-Out (FIFO)

CS-280 Dr. Mark L. Hornick 3 JCF Queue – behavioral methods offer() – place an element at the back of the queue poll() or remove() – return and remove an element from the front of the queue poll() returns null if the queue is empty remove() throws an exception if the queue is empty peek() or element() – return (without removing) the element at the front of the queue peek() returns null if the queue is empty element() throws an exception if the queue is empty

CS-280 Dr. Mark L. Hornick 4 CS2851 Review: Stack Another traditional CS data structure Classical semantics (behavior) Elements can only be inserted and removed from the front of the collection This is known as Last-In, First-Out (LIFO) Less commonly: First-Out, Last-In (FILO) Random-access is not defined

CS-280 Dr. Mark L. Hornick 5 JCF Stack – behavioral methods The naming for the structure and the methods is an analogy for how the data elements within the structure are accessed Principal methods that define behavior: push() – place an element on (top of) the stack pop() – return and remove an element from the (top of the) stack peek() – return the top element of the stack Without removing (popping) it

CS-280 Dr. Mark L. Hornick 6 Most CPUs have a built-in implementation of Stack The actual stack data structure is just an ordered collection of bytes The stack “data structure” is maintained in Data Memory A special CPU register (SP) is used to keep track of the front of the stack Dedicated instructions are used to push data (bytes) onto and pull items from the stack

CS-280 Dr. Mark L. Hornick 7 By convention, the stack starts at (or very near) the end of data memory On our Atmega32 systems, the high address in Data Memory is 0x85F Because we have 2048 (0x800) bytes of SRAM And the first 96 (0x60) bytes are assigned to Registers and IO Ports The address 0x085F is.EQU’d in m32def.inc as RAMEND 0x0000 to 0x005F 0x0060 to 0x085F Usable SRAM Mapped to Registers and IO

CS-280 Dr. Mark L. Hornick 8 The special register (SP) used to maintain the stack is called the Stack Pointer Like X, Y, and Z, the SP is a 16-bit register divided into two 8- bit registers If SP contains the address 0x085F: SPL contains the low byte of the address of the front of the stack (e.g. 0x5F) SPH contains the high byte (e.g. 0x08)......

CS-280 Dr. Mark L. Hornick 9 The SP registers are located in the IO address space In the IO address space SPL address is 0x3D SPH address is 0x3E These are.DEF’d in m32def.inc SP registers are set using the OUT instruction similar to how PORTB is written to ldi TEMP, LOW(RAMEND) out SPL,TEMP ldi TEMP,HIGH(RAMEND) outSPH, TEMP 0x085B 0x085C 0x085D 0x085E 0x085F SP

CS-280 Dr. Mark L. Hornick 10 The stack starts out with a size of 0 Data (bytes) are pushed onto the front of the stack The front of the stack grows “downward” towards lower memory address SP start Front of Stack grows downward 0x085B 0x085C 0x085D 0x085E 0x085F

CS-280 Dr. Mark L. Hornick 11 Using the Stack – pushing a byte onto the stack LDI R20, 5 ; load 5 to R20 PUSH R20 ; push 5 to stack INCR20 PUSHR20 ; push 6 to stack INCR20 PUSHR20 ; push 7 to stack SP (after 3) SP (after 2) SP (after 1) SP (before) SP is automatically decremented by each PUSH instruction 0x085B 0x085C 0x085D 0x085E 0x085F

CS-280 Dr. Mark L. Hornick 12 Using the Stack – pulling a byte from the stack POP R21 ; pull 7 from stack POPR22 ; pull 6 from stack POPR23 ; pull 5 from stack SP (before) SP (after 1) SP (after 2) SP (after 3) SP is automatically incremented by each POP instruction Although the SP is incremented, the values in Data Memory are not actually removed 0x085B 0x085C 0x085D 0x085E 0x085F

CS-280 Dr. Mark L. Hornick 13 OK, so how do you actually use the stack to do something useful? Demo

CS-280 Dr. Mark L. Hornick 14 Instructions for calling a subroutine RCALL ; relative call Like RJMP, where the call target address must be no more than +/- 2K from the call origin Operand can be a raw address (e.g. 0x0123) or a label that represents an address CALL ; long call Like JMP, where the call target address can be any address 0x0-0xFFFF ICALL; indirect call Call target address is contained indirectly in the Z register Z register can contain any address 0x0-0xFFFF The address of the next instruction following RCALL, CALL, or ICALL is automatically pushed onto the Stack

CS-280 Dr. Mark L. Hornick 15 The address of the next instruction following RCALL, CALL, or ICALL is automatically pushed onto the Stack 0x00 0x2C 0x002Aldir0,1 ; sample instruction 0x002BRCALLsub1 ; call subroutine 0x002CCLRr0 ; sample instruction SP (after RCALL) SP (before RCALL) 1. SP is automatically decremented by RCALL by 2 bytes (2-byte address) 2. The low byte is pushed first, followed by the high byte 3. Program execution jumps to the first instruction in sub1 0x085B 0x085C 0x085D 0x085E 0x085F

CS-280 Dr. Mark L. Hornick 16 Only one instruction is used for returning from a subroutine RET ; return from subroutine The return address (the address of the instruction that immediately followed the RCALL, CALL, or ICALL) is popped from the Stack and placed in the PC SP is pre-decremented during execution of RET RET is the ONLY way to return from a subroutine NEVER use RJMP or JMP to return SP will not get decremented without RET Never use JMP, RJMP, or IJMP to call a subroutine The Stack will not get incremented Use only CALL, RCALL, or ICALL

CS-280 Dr. Mark L. Hornick 17 When the RET statement in the subroutine is executed, the return address is popped from the Stack and placed in the PC 0x00 0x2C 0x002Aldir0,1 ; sample instruction 0x002BRCALLsub1 ; call subroutine 0x002CCLRr0 ; sample instruction SP (before RET) SP (after RET) SP is automatically incremented by RETby 2 bytes (2-byte address) The high byte is popped first, followed by the low byte Program execution resumes at address 0x002C after the RET from sub1 0x085B 0x085C 0x085D 0x085E 0x085F

18 Some more uses for the stack Short term data storage vs. regular Data Memory storage Easy to just PUSH and POP data on/off the stack vs. LD/ST from memory Preserving registers during calculations PUSH a register value onto the Stack to save it Do some other work involving that register POP the value off the Stack back into the register Preserving registers prior to a subroutine call PUSH register values onto the Stack to save them Call subroutine(s) that might use those registers POP the values off the Stack back into the registers after the subroutine call returns