# APSS cold boot flow

A cold boot starts the system from the power-off state. This process
begins with the PBL.

The following figure shows the cold boot flow.

![../../_images/boot-arch.png](data:image/png;base64,UklGRu4RAABXRUJQVlA4TOERAAAvHQNWAMfGNADbNpH//y8wMF3MxVuSX1DbSGrEXv+9ERODn9BXgCWFbds2lv4/OKvF/Cty27ZBTqOD/kaLBAAIKSQAey4kgJBUQR0BXBkVochqHD+0ABY+PmdiFpQql5MUgUu0Rb4fCAGBRuR7Tn/p5/qRfo57Zq9RrWdxioK2baSEP+z71h6BiJgA+/nS9M7qCYqrfP3hR3olqh7UB0EsBZBVBofyRGVWNeIkUSdJS78inlRTHhcpVQCbmZek/fjStkmRLNm2oph5Uoyr2YvIbng2YzIzz2K4mjzn8Mk8iyMnY/FVgNn0YVbtcRVR9zC3MHe3SFCT/xfRiP7DgiQ5SDVXL1ZOg3uBewmCJfxS27btjCSlbdu2bdv22LZdY9uemh3btv1p/1AjVUk13vS9x9bk6SMzEf13xLaNI9GHXO/9Jrid9y+95TmOjCSAcYg3/EdPnx+9uN7y0ctJp5oZ5yuuJp+1XbuvOFdxZXXqKZHa2XquopqCZjUJoVmVXJuUQLMixjejO4v71crLYuaehD3+eua1K7NQvbX21ZV7oa8ZtghFcw0eiyAWX9viJfkpYfXF3CPcrXo199WSwa7duXjY2h+7rcxMTk5Fh7il3spgVSuaa1jb9ZawRWYb3atJgu4uFa2u1HadgWJXQ2zuE2CZwbSa6rQc11rRXIOvoqrNd6VRAwHNrrYk21L2IdkkIAYqmplt4T6Ymc03uMW+hSdNRc3Z9i4mwFhVb1D819i/cJ9vqCbc6akdWHVnFW6uhJrCpOLaBEKNW3n0PWfhIvmKyfe1mjowqaBEKZpv8Mg0yyOWs7hctGxmQgwXhU9Dverp2mJmuU/4aFYE6gODp44zWN7wH73/9P7T+2/wZY/ChnV99xHY/xhpWoGV12jwVFLZlPtbvGCPjkQ1OBnNUwLo6KFhgbXBpwSCrMhAB+wpA8L5lEEbKE8QEM66pYIeTwAgWZ+lAhY1ORjlqeMMljf8ZwD7UTNTF9skWZvFrABqj24fKo4lly4sAz/qrHMHTXu8WZt7iXmBvb9ucqT5KN0rz3N6AC3ZbIB0QD4M6DJFAE5WAJUtYBVSqJFJICik2bFyX2rUtUOd59yLr4uwQcemsKJlRS8RoxS6HsZNZCqodGwJMpwmpB2rYhaA5AuVgB1YIRJVkn1NC4lShsrYCCWOft6utECToZdsFCrjAmgyeJAMjdCNGfslaVyyhxYVZN2/IMlSjt+CLC3obKdNoJdpt2EFs+57T8QWy7grqJBOO1ZHIYsv8mjkOX8UsgXqNsMaAwpZYUCfb5YcQEFhzTZTSFExe5/BoiZnicOc3/XaAZ46zg1+DRC4Xl7h/GJffZ4q4mc43R6uBhpKyWYfMKj/EpUCw2QiIhUsvIrskfEKFVRcryxEhb+9RdzGhEdG3Sz802uuYMKF1YeP0rkhd5zla69ou5tBdDylE4P67agTX3uvacMMouM1HR7cb0fdlH/Q9rUZRMcH2ju4347DXh0sX2g7NPq7FMON2GC6FhusVmCD3WJscJqPDe6zsMFnCjYEjseG8C5siG3FhqR6bEivwoacUmwo/I0Nf3OwoSodG+qTsIFisaErHBvGj2DDFB9smOWODfNlbPhjhw0rrLBhrSk2bDTEhu3cF2jYSx+g4TC9hoY/sf8YnuNz7wbuNWYL/sL++fhy6cJyVJsv+Hv78vGHFnGSXv6UPUabluvCmoY+GAfFlzhJL53N+PjMpuWa4qahD0Zx7wbu8zUHLV6NTXMLvNiH+83TAPcN2WPzVMHmPrXr0I+IUdy7wYyH1k++cJGf+l4RWokBzRYAHWkWAAGQ3hSgD8BGwENsHkQto7QZoqYhTOdp40lBhbFNYpQgdGySv53fSVbRJcIzs71wtk2Sl5FoJflHzVxZaYGiqKTK1GamRySY1w69xlBop3mxCd0ukCxICKss7fEEdYCPojweTMXYFhTYUOzTFEkr0MkXnp46rfFd2+gI6QBgUNpG0KMbhAr8fzycK58GS2yQrBUCNrJSldeA0M3OOa684ZPLrg12YijDs2evFKcPwCSH1V8FIR8pQq7KoWaX0ohsLb/RoIoIo46RTDwDQJShElhKC6AXKdbBKgoNrjA0ZBFDiRtAqOHDgRgpPC9AMlfudgQQ+ck3eHWo6g1kElWx/QMoQgJp2ACe/BTBZ2ilFANMTQwzIqg0B2Aeg0BUuk8YpP7W0IPRUtIndW0IeY6eRCMUV7HfMVLOweK61mnkqM8G0yGmNJTPGWgwehEjUkGRY0LLKvwi5h6apK6KvoHLPePQvaI1MwaofH//vI3riU2LWlAhrWRgg6ACVSn0kCGiZqR26CeOzw9hjKJKmgMiKH02WHbeBZDeshZSjDkyyRrxBjJBLfkNZVelNYarhBBSFFrzTv45vm/r2qXzpo1trSnJS4kN9naxMeX44iCioqeTjmgSALATAJg6BvreIpvokGjaURZ98oM7og9P3vKCNEAPQQCwgwJLCGkmNYjuqCFzNYPUvAc9MbVxkYNjU/JKalrHTpu3dO3WfcdrdVAioAhG1Qp3F8mmvtjN7WCUUcgQE5KyByNw934GlKr+V1E2mfxPuwikiFcVyQ5YkNzJ/efvvwvbWTCxIkzuxxTTdKCcS5Y8wu0YOx2gR0ZhlRspoIzGfYuussoYfaiVG2rFj4pIWc17NHQqxTZoKPLPYEQROVBhWdiUXe7MXk1GQivcWrN0H7AZAEQWGuDok8PgEuVJAvOo/bjRF2oIzUSX6eTRcJuuMHYHyySjrD2aBjUK1AsNG/UhVyUbkVZAYR4G4A9rxgojCpVmxnDHWfrz71PTzh3uEmO78EjItUc/1s4Y5hg9q7hHFxiboghQmkfcGUMBYVTu+/Fwn85LBs1t/4hhl/KA1FKic/4M4RGdhYbHdBoantDJYWJ9elM3UoCPhmd0fLjORPbBCzoGDS/pCDS84g5Dwxs6CA3vaD80vOf2iYO9llc4SGGx5RV+oL18Thd74/91ALw2DJBnBHvlI+1mIZNojyLR85l2MpAbflQq1X2h7QxkEpGNWqL7Rtt4nDKPRJMkuu+0hX3IRESRryT6itvE46R5JFJJdIYbeZw0jyTZD2y0nsdp82g256U5YxXr+IdlWV6+wCJfkOZM1jHPFnf//MXnXarHbA02mK/CBouV2GC1Ahusl2GDzVJNvvaKtrsTQsfvXvzu7vUgCS+HOGyXaPC1uND2q/O3DlHYL9ZY/kHb104IHZ+02L0epOHlEIXDQmzs7LgAG5zmY4PLXGxwnYMNbrOxwX0mNnhOxwavadjgMxUbfCdjg99EbAiYgA1B47AheCw2hI7GhrBR2BDRiQ1RHdgQ086qLk4Wm1riWpnVd08Um1zim7EhoREbkuqxIbkOG1JqsSG1GhvSq7AhsxIbsiqwIbuMjXz288ObVJRbwkL84hDWvzS3wIt9uN8MSFtku2EUnhbkFzORz9vjLqKQPTanOQVFfE5L6082oS40fRScXpXC39JHbPSSPORtCvQAWokBzRYAHWkWAAGQ3hSgD8B2UZysoWSRDCCpN0SxFFCXFKrruy9iSW7AEaeoQKMfvRC29tWVJMzstEcto2kzRE1DmM7TxpOCCmObxChB6NgkfyuBsJKeoyVZyU+WIuIykB3I2qjM7WFJQY9Kh/GIU5yv2aUL0yyL+8LzgmQVXSI8M9sLFyNTMS1RyLCys5KStpOzRBRDSWqErCSnqUIFMvFaoQePOSW5gmhbX1xLiy0t0CgqqTK1GQrzLqioPQBAYUZQageCImP96FOWLRJneCItvHboNYZCO1xRw407ClLSoVpGaNiJUFRkabnz5Q4iTiYuPOo7Rj+6BQU2FPs0RdIKdPKFp6cGgkC3BZUe0h2mZEV0KorMCIh67KnMFKMU85OJSUZZe3rPRnx8RyGBVn56JSsvlNeA0M2GwsAtUQDKZoUKD6UNQNHNjj1V6SIo5icURBQpq/tpf9eL0UwQWw6eaked6lThip1Sx+sbe8/sslT1asLO72q6k4raFME+enL8cSLFf7LoblueVNQlazsKWXwlj0ZempnZwkp9wo29+U26cS6eG1yfpGUHy8n2HRO5zyZFORcZSWBMfTZJR83xjMlafvNLkZJa49hSz1S8pKX2GLakKFJTRxQ2dEZgw6gwbBgdig1jg7FhXBCDOat/r3BCAIs5p22iHzZM9sWGqT7YMM0LG6Z7YsNMd2yY7YYNc1yxYa4LNsx3woYFjtiw0AEbFttjwxJbLXztFW2nhw7J7vVyvunwIVhqo9nX3nuh7VdPDx2Jv9i9Xs44HT4Ay/5oU/5B29dODx2i2L1ezjgdQxQrrLCx80oLbFhljg1rzLBhnQk2qIyxYb0RNmw0xIZNHDZs4b5DwzbuGzRs575Aw07uMzTs5j5qZcD++f8kRXrmmgj20gcdGuFPrkxTpGetiWEfvYeG/fQOGg5yb6DhML2ChiP0EhqOcS+g4Tg9g4aT9ETw/NuL2eV8z2l6LHi2lnSPZbbnLD0SRmuZ7VHTA+Fm/eI83Rc+//aPmvnw/HubbEO6ENCTbTN2yr1sl490CNIDwD50sKGvVhwtA8mClEKKwjZjwWMP0E8wF+ieGOWjdq0DYhvq5NpxDMWBL/IwWrIGxxLpYKVDTV3a78uAjjZ7wxRbaj6ChuNhj94ozQRzibsjBoXbqBtsWkmHfiAOfIGgnbxTh/FEOj3kMqktY6Yc9CrfB+iNKNPLFbrdr23UCFI4EIe+uNz2vmzI0UU6NY+9XtL9RmaXA+gkACpZ5JQPDrQVMDIcaDqgGYQelLGI8CDiDMBFZIbCaGFakdDmQG1FxL1U07TiDTykjlbE3kWDjm7Ty1W6JQDP5ZBJ0Y0ORe19cCgOfmHSuCc5rkgnhwqVOp2mExNXnacGS5ra2JLEKEFBQ/KXPMrtyq1eWgtlLMYelRVSJboMDZUiDT35Q2S3J51Cq6pIQc4i4ioxrfz49Pb5IbopdP7tSgPp+CAhVPQDceiLWu/Sv+/RRTp1yCSLhBrPPKQuVBpKRiIqiTpE4ZZku46AqCqZM3hGkZZTIXqkQqgRUVnRhGd3fQyTw9/9+R/9zuc+/hu/9LM/8UPymI6mmvKi3LTE6BA/Txd7K1ODnnAjonSw6MSOz0zCh+HgN0QQ3sb4Ip2+CgANt8i+PiRmIi6gyqE/YXsAALuRqzt2k+TVHDY1SHvbuz/wPZe+wVEJqTmFZdWNHWMmzZi3aPnaTTv3Hz1z8dpg4rA30yEnMj66SKfoJZH6U+NB+ugKyiHfQyjYvyCPQvGVf/mvr3/nJjOF44nCm4Ggoj/43kfk8pcPjVSu+AobVaQTtaAi97kLDRLo7V9PwDx4OGqRm0wH9iLGIlSGJxUu0Yp2Qrl/lKVXG0F4YlrdAwmSFbKVAg6DUdaerw+PtMhZeoMRRTphRyEh0g3Ryor2yv3LAoADAXW0FtJUXGpT6dSipcBWiFFoRbsxAJaHoZspvD2uDJhb6cKjQeCaTk7TwvFaU8c9Ie1OPKYp0mNrpRl5TLMvbDcjBv3xYrioN/gN/9GDb8b/Nc+hmeHtDf/Rj+XazGwf1hZDnv3zUcHljz6EwpmWxap7sXikLBZzJ5dF/+jQjbsv8nWLQCnQS1p8WYQ/K0GDJGiRGpVfSFDsiMGlvuf21ZmTtYVe0mWRv5L9I2+CrQU3AH77zn8b4HLnVoDVR4ElK0EDlq1uwK69mgil4rl/1YrkCviqi1Cqu6+S6GsYYt7ko6YWsdasLQ1hOk/DIim4ii1JjBCUlaCZJe0aL3SuXGDViwltVQpyK6Iyt5IsmFgs1GxPpmIV8sKWrAQtStKZwa+MWbctQgyLiLEwMb9WzEzj7lLL9cFYbFFcCX3in0AUobTOofizH9n6b1Yv3WkK3X8kV9wctrjhM1tm3cIX/PbNc0ufWb10tdASNwtkwjz7ka2Ej67x1ZS1D+O300dfWdb2/GZl10luzbhvfuTZXQrlOUIEx+IxezZzDwhbJwHPXsadJxGIBSEkFPePAtt8cmwR8pachN5GgCTtWbl7aasb1R1bwYiXt9oDRfCsAiE1Li6EbSX6sxO/2SnVdHnqOL/rDf/R++8/HJARBwA=)

**Figure: Cold boot flow**

The cold boot process operates in two security modes: Secure and
Nonsecure. At the EL0, EL1, and EL2 [exception
levels](https://developer.arm.com/documentation/102412/0103/Privilege-and-Exception-levels/Exception-levels),
the boot core can operate in either Secure or Nonsecure mode, however at the EL3 it
always operates in Secure mode. The Linux kernel runs in the nonsecure EL1 state. After completing the security settings in the secure EL3 state, the XBL may switch to the UEFI nonsecure EL1 state.

For more information about the security modes, see [Software Security
Architecture](https://docs.qualcomm.com/bundle/publicresource/topics/80-70018-11/architecture.html).

The cold boot sequence involves the following steps:

1. After a reset, the boot core exits the Reset state and executes the
PBL. The PBL initializes the hardware clocks, CPU caches, memory
management unit (MMU), and identifies the boot device based on the
boot option settings.
2. The PBL loads and authenticates the XBL from the boot device. The XBL
runs the security setup in EL3 Secure state and performs the
following tasks:

    1. Initializes the hardware, firmware images, CPU cache, MMU, boot
device, PMIC, and DDR.
    2. Loads and authenticates the Qualcomm TEE image from the boot
device.
    3. Loads and authenticates the Qualcomm Hypervisor image.
    4. Loads and authenticates the UEFI image.
    5. Makes a secure channel manager (SCM) call to jump to the Qualcomm
TEE image. SCM is the driver that communicates with Qualcomm TEE
using a [secure monitor
call](https://developer.arm.com/documentation/ddi0333/latest/programmer-s-model/exceptions/secure-monitor-call--smc-)
(SMC).
3. The Qualcomm TEE sets up the secure environment and runs the Qualcomm Hypervisor image.
4. The Qualcomm Hypervisor hands over control to UEFI, which then loads and authenticates the systemd-boot EFI image using the [UEFI secure
boot](https://docs.qualcomm.com/bundle/publicresource/topics/80-70018-11/enable-uefi-secure-boot.html).
5. The systemd-boot image loads and authenticates the Qualcomm Linux kernel image, and passes control to the Qualcomm Linux kernel. If you enable kernel-based virtual machine (KVM) mode, the UEFI shuts down the Qualcomm Hypervisor, exits the UEFI boot services, and passes control to Linux KVM in EL2.
6. The Qualcomm Linux kernel launches the Linux application such as
the bash shell.

For Reat time subsystem (RTSS) cold boot flow, which runs independently and in parallel to the APSS boot flow, see [RTSS cold boot flow](https://docs.qualcomm.com/doc/80-70018-4/topic/Sail.html#sail).

Last Published: Apr 30, 2025

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