Chapter 13
- The compiler generates the same assembly language for the
whileandforloops. The difference is only in the C syntax. Both thewhileandforloops branch to the bottom of the loop body, where the loop termination condition is tested before branching to the top of the loop body:b .L2 .L3: mov x2, 1 ldr x1, [sp, 24] mov w0, 1 bl write ldr x0, [sp, 24] add x0, x0, 1 str x0, [sp, 24] .L2: ldr x0, [sp, 24] ldrb w0, [x0] cmp w0, 0 bne .L3The
do-whileloop executes the loop body before testing the loop termination condition:.L2: mov x2, 1 ldr x1, [sp, 24] mov w0, 1 bl write ldr x0, [sp, 24] add x0, x0, 1 str x0, [sp, 24] ldr x0, [sp, 24] ldrb w0, [x0] cmp w0, 0 bne .L2 - Program to echo characters entered on keyboard.
// Echo characters entered on keyboard. .arch armv8-a // Useful names .equ MAX, 10 .equ NUL, 0 .equ LF, 10 // '\n' in Linux .equ STDIN, 0 .equ STDOUT, 1 // Stack frame .equ save1920, 16 .equ the_string, 32 .equ FRAME, 48 // Constant data .section .rodata prompt: .string "Enter up to 10 characters: " response: .string "You entered: " newline: .string "\n" // Code .text .align 2 .global main .type main, %function main: stp fp, lr, [sp, -FRAME]! // Create stack frame mov fp, sp // Set our frame pointer stp x19, x20, [sp, save1920] // Save for caller adr x19, prompt // Address of prompt message prompt_loop: ldrb w0, [x19] // Load character cmp w0, NUL // End of string? b.eq read_input // Yes mov w2, 1 // No, one char mov x1, x19 // Address of char mov x0, STDOUT // Write on screen bl write add x19, x19, 1 // Increment pointer b prompt_loop // and continue read_input: add x19, sp, the_string // Address of string storage mov w20, wzr // Zero characters stored read_loop: mov w2, 1 // One char mov x1, x19 // Place to store it mov x0, STDIN // Read from keyboard bl read ldrb w0, [x19] // Load character cmp w0, LF // End of input? b.eq terminate_string // Yes cmp w20, MAX // No, is storage array full? b.ge read_loop // Yes, read but don't keep add x19, x19, 1 // Increment pointer add x20, x20, 1 // and counter b read_loop // No, continue reading terminate_string: mov w0, NUL // Yes, string termination char strb w0, [x19] // Terminate the string adr x19, response // Address of response message response_loop: ldrb w0, [x19] // Load character cmp w0, NUL // End of string? b.eq echo // Yes mov w2, 1 // No, one char mov x1, x19 // Address of char mov x0, STDOUT // Write on screen bl write add x19, x19, 1 // Increment pointer b response_loop // and continue echo: add x19, sp, the_string // Get beginning of string storage echo_loop: ldrb w0, [x19] // Load character cmp w0, NUL // End of string? b.eq done // Yes mov w2, 1 // No, one char mov x1, x19 // Address of char mov x0, STDOUT // Write on screen bl write add x19, x19, 1 // Increment pointer b echo_loop // and continue done: mov w2, 1 // One char adr x1, newline // Address of newline char mov x0, STDOUT // Write on screen bl write mov w0, wzr // Return 0 ldp x19, x20, [sp, save1920] // Restore regs ldp fp, lr, [sp], FRAME // Delete stack frame ret - Top and bottom fourths are heads.
// Flip a coin, showing heads or tails. .arch armv8-a // Useful names .equ N_TIMES, 10 // Number of flips .equ RAND_BOTTOM, 536870911 // RAND_MAX/4 .equ RAND_TOP, 1610612733 // 3*RAND_MAX/4 // Stack frame .equ save19, 28 .equ FRAME, 32 // Constant data .section .rodata heads_msg: .string "heads" tails_msg: .string "tails" // Code .text .align 2 .global main .type main, %function main: stp fp, lr, [sp, -FRAME]! // Create stack frame mov fp, sp // Set our frame pointer str w19, [sp, save19] // Save for i local var mov w19, wzr // i = 0 loop: mov w0, N_TIMES // Total number of times cmp w19, w0 // Is i at end? b.hs done // Yes bl random // No, get random number mov w1, RAND_TOP & 0xffff // Top fourth; needs movk w1, RAND_TOP / 0x10000, lsl 16 // two instructions cmp w1, w0 // Above or below? b.lo heads // Above -> heads mov w1, RAND_BOTTOM & 0xffff // Bottom fourth; needs movk w1, RAND_BOTTOM / 0x10000, lsl 16 // two instructions cmp w1, w0 // Above or below? b.hi heads // Below -> heads adr x0, tails_msg // Middle -> tails bl puts // Print message b continue // Skip else part heads: adr x0, heads_msg // Heads message address bl puts // Print message continue: add w19, w19, 1 // Increment i b loop // and continue loop done: mov w0, wzr // Return 0 ldr w19, [sp, save19] // Restore reg ldp fp, lr, [sp], FRAME // Delete stack frame ret // Back to caller - The assembly language shows that the
bto the default case at the end of cases 1, 2, and 3 is removed when thebreak;C statement is deleted. The switch is entered at the same point in the list of statements but all the subsequent statements are also executed:$ ./switch_nb i = 1 i = 2 i = 3 i > 3 i = 2 i = 3 i > 3 i = 3 i > 3 i > 3 i > 3 i > 3 i > 3 i > 3 i > 3 i > 3 ifstatements.// Select one of three or default. .arch armv8-a // Useful names .equ NTIMES, 10 // number of loops // Stack frame .equ save19, 16 .equ FRAME, 32 // Constant data .section .rodata one_msg: .string "i = 1" two_msg: .string "i = 2" three_msg: .string "i = 3" over_msg: .string "i > 3" // Program code .text .align 2 .global main .type main, %function main: stp fp, lr, [sp, -FRAME]! // Create stack frame mov fp, sp // Set our frame pointer str x19, [sp, save19] // Save reg mov w19, 1 // i = 1 loop: cmp x19, NTIMES // Is i at end? b.hi all_done // Yes, leave loop cmp w19, 1 // One? b.ne two // No adr x0, one_msg // Yes, tell user bl puts two: cmp w19, 2 // Two? b.ne three // No adr x0, two_msg // Yes bl puts three: cmp w19, 3 // Three? b.ne over // No adr x0, three_msg // Yes bl puts over: cmp w19, 3 // Less than 3? b.le less // Yes adr x0, over_msg bl puts less: add w19, w19, 1 // i++ b loop // And continue loop all_done: mov w0, wzr // Return 0 ldr x21, [sp, save19] // Restore reg ldp fp, lr, [sp], FRAME // Delete stack frame ret ```asmif-elseladder.// Select one of three or default. .arch armv8-a // Useful names .equ NTIMES, 10 // number of loops // Stack frame .equ save19, 16 .equ FRAME, 32 // Constant data .section .rodata one_msg: .string "i = 1" two_msg: .string "i = 2" three_msg: .string "i = 3" over_msg: .string "i > 3" // Program code .text .align 2 .global main .type main, %function main: stp fp, lr, [sp, -FRAME]! // Create stack frame mov fp, sp // Set our frame pointer str x19, [sp, save19] // Save reg mov w19, 1 // i = 1 loop: cmp x19, NTIMES // Is i at end? b.hi all_done // Yes, leave loop cmp w19, 1 // One? b.ne two // No adr x0, one_msg // Yes, tell user bl puts b continue two: cmp w19, 2 // Two? b.ne three // No adr x0, two_msg // Yes bl puts b continue three: cmp w19, 3 // Three? b.ne over // No adr x0, three_msg // Yes bl puts b continue over: adr x0, over_msg bl puts continue: add w19, w19, 1 // i++ b loop // And continue loop all_done: mov w0, wzr // Return 0 ldr x21, [sp, save19] // Restore reg ldp fp, lr, [sp], FRAME // Delete stack frame ret