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19619: cpu/arm7: Fix undefined behavior based on invalid assembly r=aabadie a=Teufelchen1 ### Contribution description Hellooo! 🦤 In this PR a single `NOP` is added after an `ldm` in ARM7 common code in the scheduling management. The change is necessary because this particular `ldm` affects certain banked registers, including `R14 / lr`. The next instruction then immediately accesses `lr`. This is invalid and the exact effect depends on the underlying hardware implementation. An intermediate `NOP` ensures correct behaviour in such cases. The ARM docs can be found [here](https://documentation-service.arm.com/static/5f8dacc8f86e16515cdb865a), in section `A4.1.21 LDM (2)`. It states: > In ARM architecture versions earlier than ARMv6, this form of LDM must not be followed by an instruction that accesses banked registers. A following NOP is a good way to ensure this. ### Testing procedure Without this change, on the Game Boy Advance, RIOT behaves irregularly. With the `NOP`, it works fine! If possible, this change should be tested on other ARM7 hardware as well but I do not have access to any. ---- Special thanks `@pyropeter` for spotting the extremely tiny note on this issue within the ARM docs. You saved me hours of tears! 👾 Co-authored-by: Teufelchen1 <bennet.blischke@outlook.com> |
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| .. | ||
| arm7_common | ||
| atmega32u4 | ||
| atmega128rfa1 | ||
| atmega256rfr2 | ||
| atmega328p | ||
| atmega1281 | ||
| atmega1284p | ||
| atmega2560 | ||
| atmega_common | ||
| atxmega | ||
| avr8_common | ||
| cc26x0_cc13x0 | ||
| cc26x2_cc13x2 | ||
| cc26xx_cc13xx | ||
| cc2538 | ||
| cortexm_common | ||
| efm32 | ||
| esp32 | ||
| esp8266 | ||
| esp_common | ||
| fe310 | ||
| gd32v | ||
| kinetis | ||
| lm4f120 | ||
| lpc23xx | ||
| lpc1768 | ||
| msp430_common | ||
| msp430fxyz | ||
| native | ||
| nrf5x_common | ||
| nrf51 | ||
| nrf52 | ||
| nrf9160 | ||
| qn908x | ||
| riscv_common | ||
| rpx0xx | ||
| sam0_common | ||
| sam3 | ||
| sam_common | ||
| samd5x | ||
| samd21 | ||
| saml1x | ||
| saml21 | ||
| stellaris_common | ||
| stm32 | ||
| doc.txt | ||
| Kconfig | ||