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added and documented example code

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Perry Kivolowitz 2022-07-13 12:41:26 -05:00
parent 2614ef9ae4
commit 22cbb583b1
2 changed files with 71 additions and 11 deletions
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41
section_2/bitfields/review.md
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@ -117,6 +117,8 @@ specified bits from the source. The `u` means don't consider the
sign bit as special. The `z` is what causes the zero fill first and sign bit as special. The `z` is what causes the zero fill first and
sets it apart from `bfi`. sets it apart from `bfi`.
See bottom for an example.
## `mvn` ## `mvn`
This instruction takes only takes two operands (but permits an optional This instruction takes only takes two operands (but permits an optional
@ -147,6 +149,8 @@ That is, if your goal is to turn a positive integer into a negative
integer, use `neg`. If your goal is to negate bits for bit bashing integer, use `neg`. If your goal is to negate bits for bit bashing
purposes, use `mvn`.* purposes, use `mvn`.*
See bottom for an example.
## `lsl` ## `lsl`
**Logical Shift Left** moves bits to the left shifting 0 into any **Logical Shift Left** moves bits to the left shifting 0 into any
@ -175,3 +179,40 @@ for bit bashing as it is how bits can be set to 1.
orr rd, rs, rm # rm is another register orr rd, rs, rm # rm is another register
orr rd, rs, imm orr rd, rs, imm
``` ```
## Example
[Here](./ubfiz.s) is the code to an example:
```asm
.global main // 1
.text // 2
.align 2 // 3
/* This demo should be run from gdb. `layout regs` would be helpful. // 4
This demo shows: // 5
bfi - bit field insert // 6
*/ // 7
main: mov w1, wzr // // 8
mvn w1, w1 // set w1 to 0xFFFFFFFF // 9
mov w3, w1 // save for reuse // 10
mov w2, 3 // set bits 0 and 1 // 11
bfi w1, w2, 4, 4 // 12
// Look at w1. You will note that the bottom // 13
// 4 bits of w2 (0011) have been copied into the second // 14
// 4 bits of w1 including the zeros. // 15
// // 16
// NEXT DEMO // 17
mov w1, w3 // reset w1 // 18
mov w2, 3 // set bits 0 and 1 // 19
ubfiz w1, w2, 4, 2 // 20
// Look at w1. You will note that all the bits were // 21
// zeroed and then 2 bits from w2 are copied into w1 // 22
// starting at bit 4. Compare this to bfi. // 23
// // 24
// NEXT DEMO // 25
bfxil w3, w1, 4, 4 // 26
// Look at w3. You will note that 4 bits (0011) from // 27
// w1 were put into w3 starting at bit 0. // 28
mov w0, wzr // 29
ret // 30
```

41
section_2/bitfields/ubfiz.s
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@ -1,11 +1,30 @@
.global main .global main
.text .text
.align 2 .align 2
/* This demo should be run from gdb. `layout regs` would be helpful.
main: str x30, [sp, -16]! This demo shows:
mov w1, 0xFF bfi - bit field insert
mov w2, 0x0A */
ubfiz w1, w2, 0, 4 main: mov w1, wzr //
ldr x30, [sp], 16 mvn w1, w1 // set w1 to 0xFFFFFFFF
mov w0, wzr mov w3, w1 // save for reuse
ret mov w2, 3 // set bits 0 and 1
bfi w1, w2, 4, 4
// Look at w1. You will note that the bottom
// 4 bits of w2 (0011) have been copied into the second
// 4 bits of w1 including the zeros.
//
// NEXT DEMO
mov w1, w3 // reset w1
mov w2, 3 // set bits 0 and 1
ubfiz w1, w2, 4, 2
// Look at w1. You will note that all the bits were
// zeroed and then 2 bits from w2 are copied into w1
// starting at bit 4. Compare this to bfi.
//
// NEXT DEMO
bfxil w3, w1, 4, 4
// Look at w3. You will note that 4 bits (0011) from
// w1 were put into w3 starting at bit 0.
mov w0, wzr
ret