Wat gaan we doen?

1. Wat gaan we doen? • harhaling data types • herhaling memory adressing modes • gebruik van de stack • load/store multiple instructions • uitleg SET_LEDS • uitleg Kitt Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

2. ARM7 data types • Word is 32 bits long. • Half word is 16-bits long (ARM7TDMI) • Word can be divided into four bytes. • ARM addresses 32 bits. • Address refers to byte. • Address 4 starts at byte 4. • Can be configured at power-up as either little- or big-endian mode. Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

3. Little- and big-endian storage r0 = 0x11223344 11 22 33 44 STR r0,[r1] 3 2 1 0 0 1 2 3 11 22 33 44 11 22 33 44 LDRB r2,[r1] 00 00 00 44 00 00 00 11 little-endian big-endian Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

4. Memory r0 SourceRegisterfor STR 0x5 r1 r2 DestinationRegisterfor LDR BaseRegister 0x200 0x5 0x200 0x5 Addressing mode: Base Register • The memory location to be accessed is held in a base register • STR r0, [r1] ; Store contents of r0 to location pointed to ; by contents of r1. • LDR r2, [r1] ; Load r2 with contents of memory location ; pointed to by contents of r1. Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

5. r0 SourceRegisterfor STR Memory 0x5 Offset 12 0x5 0x20c r1 BaseRegister 0x200 0x200 Addressing mode: Pre-indexed • Example: STR r0, [r1,#12] • To store to location 0x1f4 instead use: STR r0, [r1,#-12] • To auto-increment base pointer to 0x20c use: STR r0, [r1, #12]! • If r2 contains 3, access 0x20c by multiplying this by 4: • STR r0, [r1, r2, LSL #2] Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

6. Memory r0 r1 Offset SourceRegisterfor STR UpdatedBaseRegister 0x5 0x20c 12 0x20c 0x5 0x200 r1 OriginalBaseRegister 0x200 Addressing mode: Post-indexed • Example: STR r0, [r1], #12 • To auto-increment the base register to location 0x1f4 instead use: • STR r0, [r1], #-12 • If r2 contains 3, auto-incremenet base register to 0x20c by multiplying this by 4: • STR r0, [r1], r2, LSL #2 Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

7. assembler instructie formaat : multiple words van en naar geheugen (block transfer instructies) STMIA R0, { R1-R9 } STMIA R0!, { R1-R9 } STMIB R0, { R1-R9 } ; DA, DB STMNEIA R0, { R1-R9 } Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

8. 24 23 22 21 20 19 31 28 27 16 15 0 Block Data Transfer instructie code • The Load and Store Multiple instructions (LDM / STM) allow betweeen 1 and 16 registers to be transferred to or from memory. • The transferred registers can be either: • Any subset of the current bank of registers (default). • Any subset of the user mode bank of registers when in a priviledged mode (postfix instruction with a ‘^’). Cond 1 0 0 P U S W L Rn Register list Base register Condition field Each bit corresponds to a particular register. For example: • Bit 0 set causes r0 to be transferred. • Bit 0 unset causes r0 not to be transferred. At least one register must be transferred as the list cannot be empty. Up/Down bit 0 = Down; subtract offset from base 1 = Up ; add offset to base Load/Store bit 0 = Store to memory 1 = Load from memory Write- back bit 0 = no write-back 1 = write address into base Pre/Post indexing bit 0 = Post; add offset after transfer,1 = Pre ; add offset before transfer PSR and force user bit 0 = don’t load PSR or force user mode 1 = load PSR or force user mode Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

9. PUSH {1,2,3} 3 SP 2 SP 1 BASE Stack • A stack is an area of memory which grows as new data is “pushed” onto the “top” of it, and shrinks as data is “popped” off the top. • Two pointers define the current limits of the stack. • A base pointer • used to point to the “bottom” of the stack (the first location). • A stack pointer • used to point the current “top” of the stack. POP Result of pop = 3 2 1 SPBASE BASE Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

10. subroutine call and return BLX subroutine ; continue here subroutine: ; do something MOV PC, LR Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

11. Stacks and Subroutines • One use of stacks is to create temporary register workspace for subroutines. STMFD sp!,{r0-r12, lr} ; stack all registers ........ ; and the return address ........ LDMFD sp!,{r0-r12, pc} ; load all the registers ; and return automatically • See the chapter on the ARM Procedure Call Standard in the SDT Reference Manual for further details of register usage within subroutines. • If the pop instruction also had the ‘S’ bit set then the transfer of the PC when in a priviledged mode would also cause the SPSR to be copied into the CPSR (see exception handling module). Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

12. Stack Operation • Traditionally, a stack grows down in memory, with the last “pushed” value at the lowest address. The ARM also supports ascending stacks, where the stack structure grows up through memory. • The value of the stack pointer can either: • Point to the last occupied address (Full stack) • and so needs pre-decrementing (ie before the push) • Point to the next occupied address (Empty stack) • and so needs post-decrementing (ie after the push) • The stack type to be used is given by the postfix to the instruction: • STMFD / LDMFD : Full Descending stack • STMFA / LDMFA : Full Ascending stack. • STMED / LDMED : Empty Descending stack • STMEA / LDMEA : Empty Ascending stack • Note: ARM Compiler will always use a Full descending stack. Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

13. SET_LEDS SET_LEDS: @ save registers stmfd sp!, { r0-r2, lr } @ set LEDs that must be turned on mov r0, r0, LSL #8 ldr r1, =IOSET str r0, [ r1 ] @ clear LEDs that must be turned off mvn r0, r0 ldr r1, =IOCLR str r0, [ r1 ] @ save registers and return ldmfd sp!, { r0-r2, pc } Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

14. Kitt @ initialisation ldr r2, =1 @ rightmost LED on ldr r3, =1 @ means: shift left loop: cmp r3, #1 moveq r2, r2, LSL #1 movne r2, r2, LSR #1 cmp r2, #1 moveq r3, #1 cmp r2, #0x80 moveq r3, #0 b loop Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology

15. doen • Kitt afmaken • Toon aan hoe snel • een interne instructie uitgevoerd wordt • een I/O instructie uitgevoerd wordt • Maak een ‘echte’ Delay_uS subroutine Hogeschool van Utrecht / Institute for Computer, Communication and Media Technology