EE 308: Microcontrollers

Transcription

1 EE 308: Microcontrollers Assmbly Language Part I Aly El-Osery Electrical Engineering Department New Mexico Institute of Mining and Technology Socorro, New Mexico, USA January 30, 2018 Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

2 Introduction to Assembly In the Introduction to Assembly lecture we discussed Structure of the assembly language Op Codes Instructions: LDI, ADD, LDS, STS, IN and OUT Assembling, linking and downloading Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

3 Example 1 Write a list of instructions that stores the numbers 32, 45 and 15 in memory locations 0x200, 0x201 and 0x202, respectively Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

4 Example 2 Write a list of instructions that adds the numbers 32, 45 and 15 Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

5 Example 3 Write a list of instructions that adds the numbers in memory locations 0x200, 0x201 and 0x202 and store the result in memory location 0x203 Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

6 Register usage convention Call-used registers (r18-r27, r30-r31): May be allocated for local data. You may use them freely in assembler subroutines. Calling subroutines can clobber any of them and the caller is responsible for saving and restoring. Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

7 Register usage convention Call-used registers (r18-r27, r30-r31): May be allocated for local data. You may use them freely in assembler subroutines. Calling subroutines can clobber any of them and the caller is responsible for saving and restoring. Call-saved registers (r2-r17, r28-r29): May be allocated for local data. Calling subroutines leaves them unchanged. Assembler subroutines are responsible for saving and restoring these registers, if changed. Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

8 Register usage convention Call-used registers (r18-r27, r30-r31): May be allocated for local data. You may use them freely in assembler subroutines. Calling subroutines can clobber any of them and the caller is responsible for saving and restoring. Call-saved registers (r2-r17, r28-r29): May be allocated for local data. Calling subroutines leaves them unchanged. Assembler subroutines are responsible for saving and restoring these registers, if changed. Fixed registers (r0, r1): Never allocated by gcc for local data, but often used for fixed purposes: Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

9 Register usage convention Call-used registers (r18-r27, r30-r31): May be allocated for local data. You may use them freely in assembler subroutines. Calling subroutines can clobber any of them and the caller is responsible for saving and restoring. Call-saved registers (r2-r17, r28-r29): May be allocated for local data. Calling subroutines leaves them unchanged. Assembler subroutines are responsible for saving and restoring these registers, if changed. Fixed registers (r0, r1): Never allocated by gcc for local data, but often used for fixed purposes: r0 - temporary register, can be clobbered by any code (except interrupt handlers which save it), may be used to remember something for a while within one piece of assembler code Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

10 Register usage convention Call-used registers (r18-r27, r30-r31): May be allocated for local data. You may use them freely in assembler subroutines. Calling subroutines can clobber any of them and the caller is responsible for saving and restoring. Call-saved registers (r2-r17, r28-r29): May be allocated for local data. Calling subroutines leaves them unchanged. Assembler subroutines are responsible for saving and restoring these registers, if changed. Fixed registers (r0, r1): Never allocated by gcc for local data, but often used for fixed purposes: r0 - temporary register, can be clobbered by any code (except interrupt handlers which save it), may be used to remember something for a while within one piece of assembler code r1 - assumed to be always zero in any code, may be used to remember something for a while within one piece of assembler code, but must then be cleared after use (clr r1). Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

11 Register usage convention Call-used registers (r18-r27, r30-r31): May be allocated for local data. You may use them freely in assembler subroutines. Calling subroutines can clobber any of them and the caller is responsible for saving and restoring. Call-saved registers (r2-r17, r28-r29): May be allocated for local data. Calling subroutines leaves them unchanged. Assembler subroutines are responsible for saving and restoring these registers, if changed. Fixed registers (r0, r1): Never allocated by gcc for local data, but often used for fixed purposes: r0 - temporary register, can be clobbered by any code (except interrupt handlers which save it), may be used to remember something for a while within one piece of assembler code r1 - assumed to be always zero in any code, may be used to remember something for a while within one piece of assembler code, but must then be cleared after use (clr r1). In other words when in doubt, you can use r18-r25 freely Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

12 Purpose Control the flow of the program Conditional branch, e.g., BREQ, BRNE, BRBS Unconditional branching: JMP, IJMP Branching to a subroutine Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

13 Conditional branching: BRNE instruction Conditional relative branch. BRNE k ;Branch if not equal k is a relative location If Rd Rr (Z=0) then PC PC+k+1, else PC PC+1 Examples: a1:... CPI r27,5 ;Compare r27 to 5 BRNE a1 ;load R20 with immediate hex value 0x3F Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

14 Key points Conditional branching instructions are of the form XXXX XXKK KKKK KXXX X means don t care k is the address to branch to relative to the program counter of the next instruction k can be either positive or negative What is the range of k? Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

15 Unconditional branching: JMP instruction Long jump JMP k ;jump to address k K KKKK 110K KKKK XXKK KKKK KKKK k is a relative location 4-byte instruction to address any memory location in 4M address space Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

16 Unconditional branching: RJMP instruction Long jump RJMP k ;jump to address k 1100 KKKK KKKK KKKK k is a relative location 2-byte instruction PC PC+k+1 What is the range of k? Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

17 Repeat a group of instructions Similar to for loop Increment or decrement a register and branch as long a condition is satisfied Using instructions like DEC or INC May have to use nested loop due to the size of the register (what is the maximum using for example R16?) Used quite a bit for creating delays Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

18 Looping example L1: ;start of the loop... ;some statement... ;another statement DEC Rn ;decrement Rn (What is the value of Z?) BRNE L1 ;branch to L1 if Z =? Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13

19 Looping example L1: ;start of the loop... ;some statement... ;another statement DEC Rn ;decrement Rn (What is the value of Z?) BRNE L1 ;branch to L1 if Z =? How can we use the above approach to create delays? Aly El-Osery (NMT) EE 308: Microcontrollers January 30, / 13