Sojus07/hackrf
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Keep buffer base & size mask in high registers.

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The high registers (r8-r14) cannot be used directly by most of the
instructions in the Cortex-M0 instruction set.

One of the few instructions that can use them is mov, which can use
any pair of registers.

This allows saving two cycles, by replacing two loads (2 cycles each)
with moves (1 cycle each), after stashing the required values in high
registers at startup.
This commit is contained in:
Martin Ling
2021-12-20 12:58:10 +00:00
parent 8f43dc1be5
commit 2f26ebffd4
1 changed files with 10 additions and 4 deletions
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14
firmware/hackrf_usb/sgpio_m0.s
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@ -66,8 +66,8 @@ shadow registers.
There are two key code paths, with the following worst-case timings: There are two key code paths, with the following worst-case timings:
RX: 151 cycles RX: 149 cycles
TX: 136 cycles TX: 134 cycles
Design Design
====== ======
@ -120,6 +120,8 @@ registers and fixed memory addresses.
/* Allocations of single-use registers */ /* Allocations of single-use registers */
buf_base .req r12
buf_mask .req r11
sgpio_data .req r7 sgpio_data .req r7
sgpio_int .req r6 sgpio_int .req r6
buf_ptr .req r5 buf_ptr .req r5
@ -134,6 +136,10 @@ main:
// Initialise registers used for constant values. // Initialise registers used for constant values.
ldr sgpio_int, =SGPIO_EXCHANGE_INTERRUPT_BASE // 2 ldr sgpio_int, =SGPIO_EXCHANGE_INTERRUPT_BASE // 2
ldr sgpio_data, =SGPIO_SHADOW_REGISTERS_BASE // 2 ldr sgpio_data, =SGPIO_SHADOW_REGISTERS_BASE // 2
ldr r0, =TARGET_DATA_BUFFER // 2
mov buf_base, r0 // 1
ldr r0, =TARGET_BUFFER_MASK // 2
mov buf_mask, r0 // 1
loop: loop:
// The worst case timing is assumed to occur when reading the interrupt // The worst case timing is assumed to occur when reading the interrupt
// status register *just* misses the flag being set - so we include the // status register *just* misses the flag being set - so we include the
@ -164,7 +170,7 @@ loop:
str r1, [sgpio_int, #INT_CLEAR] // 8 str r1, [sgpio_int, #INT_CLEAR] // 8
// ... and grab the address of the buffer segment we want to write to / read from. // ... and grab the address of the buffer segment we want to write to / read from.
ldr r0, =TARGET_DATA_BUFFER // r0 = &buffer // 2 mov r0, buf_base // r0 = &buffer // 1
ldr r3, =TARGET_BUFFER_POSITION // r3 = &position_in_buffer // 2 ldr r3, =TARGET_BUFFER_POSITION // r3 = &position_in_buffer // 2
ldr r2, [r3] // r2 = position_in_buffer // 2 ldr r2, [r3] // r2 = position_in_buffer // 2
add buf_ptr, r0, r2 // buf_ptr = &buffer + position_in_buffer // 1 add buf_ptr, r0, r2 // buf_ptr = &buffer + position_in_buffer // 1
@ -212,7 +218,7 @@ direction_rx:
done: done:
// Finally, update the buffer location... // Finally, update the buffer location...
ldr r0, =TARGET_BUFFER_MASK // 2 mov r0, buf_mask // 1
and r0, buf_ptr, r0 // r0 = (pos_in_buffer + size_copied) % buffer_size // 1 and r0, buf_ptr, r0 // r0 = (pos_in_buffer + size_copied) % buffer_size // 1
// ... restore &position_in_buffer, and store the new position there... // ... restore &position_in_buffer, and store the new position there...