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https://github.com/pkivolowitz/asm_book.git
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108 lines
3.1 KiB
ArmAsm
108 lines
3.1 KiB
ArmAsm
.global main
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.align 2
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.text
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.equ NUM_FLAKES, 150
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.equ MAX_COLUMN, 80
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.equ MAX_LINE, 24
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.equ MAX_LINE_X2, 48
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storm .req x27
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main: stp x29, x30, [sp, -16]!
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stp x27, x28, [sp, -16]!
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// Seed the RNG with the current time.
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mov w0, wzr
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bl time
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bl srand
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// Allocate the storm.
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mov x0, NUM_FLAKES // Calculate space needed.
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mov x1, Flake.size
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mul x0, x0, x1
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str x0, [sp, -16]! // Save for memset.
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bl malloc // Attempt the allocation.
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cbnz x0, 1f // Branch if successful.
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add sp, sp, 16 // Undo stack change.
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ldr x0, =bad_malloc // Inform user of tragedy.
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bl puts
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mov w0, 1
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b bye
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1: mov storm, x0 // The base address of the
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// allocated memory will be
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// preserved in x27.
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ldr x2, [sp], 16 // Restore the allocation size.
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mov w1, wzr // Fill with zero.
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bl memset // x0 still had base address.
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99: mov x0, storm
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bl free
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mov w0, wzr
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bye: ldp x27, x28, [sp], 16
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ldp x20, x30, [sp], 16
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ret
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/* x0 contains the address of a Flake in need of being reset.
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The column number of a Flake is anywhere from 1 to 80 (a
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default terminal). The starting line is anywhere from 1 to
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48 (24 x 2) lines ABOVE the screen. This allows the flakes to
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drift downward without apparent gaps or bunches.
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*/
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ResetFlake:
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stp x20, x30, [sp, -16]!
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mov x20, x0
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// Initialize Column --- 1 <= random value <= 80
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bl rand
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mov x1, MAX_COLUMN
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bl mod
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add w1, w1, 1
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str w1, [x20, 4]
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// Initialize Line --- -(1 <= random value <= 48)
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bl rand
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mov x1, MAX_LINE_X2
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bl mod
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add w1, w1, 1
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neg w1, w1
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str w1, [x20]
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ldp x20, x30, [sp], 16
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ret
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/* mod(a, b) - implements a % b - AARCH64 lacks a mod instruction.
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A strange place to economize, but there you have it.
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a comes to us in x0
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b comes to us in x1
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method:
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* integer divide a by b
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for example - 5 % 3 would need 5 // 3 yielding 1
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* multiply result by b
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1 * 3 is 3
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* subtract result from a
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5 - 3 is 2 and that's our return value.
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*/
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mod: sdiv x2, x0, x1 // x2 gets a // b
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msub x0, x2, x1, x0 // x0 gets a - a // b * b
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ret
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.data
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bad_malloc: .asciz "Allocation of flakes has failed"
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.section Flake
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.struct 0
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Flake.line: .struct Flake.line + 4
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Flake.col: .struct Flake.col + 4
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Flake.size:
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