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https://github.com/pkivolowitz/asm_book.git
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83 lines
2.2 KiB
ArmAsm
83 lines
2.2 KiB
ArmAsm
.text
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.align 4
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.global main
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main: str x30, [sp, -16]!
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mov x0, xzr // set up call to time(nullptr)
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bl time // call time setting up srand
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bl srand // call srand setting up rand
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bl rand // get a random number
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and x0, x0, 7 // ensure its range is 0 to 7
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// note use of x register is on purpose
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lsl x0, x0, 2 // multiply by 4
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ldr x1, =jt // load base address of jump table
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add x1, x1, x0 // add offset to base address
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br x1
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// If, as in this case, all the "cases" have the same number of
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// instructions then this intermediate jump table can be omitted saving
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// some space and a tiny amount of time. To omit the intermediate jump
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// table, you'd multiply by 12 above and not 4. Twelve because each
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// "case" has 3 instructions (3 x 4 == 12).
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// Question for you: If you did omit the jump table, relative to what
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// would you jump (since "jt" would be gone).
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jt: b 0f
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b 1f
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b 2f
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b 3f
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b 4f
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b 5f
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b 6f
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b 7f
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0: ldr x0, =ZR
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bl puts
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b 99f
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1: ldr x0, =ON
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bl puts
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b 99f
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2: ldr x0, =TW
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bl puts
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b 99f
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3: ldr x0, =TH
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bl puts
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b 99f
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4: ldr x0, =FR
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bl puts
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b 99f
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5: ldr x0, =FV
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bl puts
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b 99f
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6: ldr x0, =SX
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bl puts
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b 99f
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7: ldr x0, =SV
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bl puts
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b 99f
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99: mov w0, wzr
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ldr x30, [sp], 16
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ret
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.data
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.section .rodata
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ZR: .asciz "0 returned"
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ON: .asciz "1 returned"
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TW: .asciz "2 returned"
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TH: .asciz "3 returned"
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FR: .asciz "4 returned"
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FV: .asciz "5 returned"
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SX: .asciz "6 returned"
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SV: .asciz "7 returned"
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.end
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