learn more formally arm 32 bit CP registers, fix QEMU baremetal run with cli args

README.adoc

@@ -21486,6 +21486,35 @@ Guaranteed undefined! Therefore raise illegal instruction signal. Used by GCC `_
 
 Why GNU GAS 2.29 does not have a mnemonic for it in A64 because it is very recent: shows in <<armarm8-db>> but not `ca`.
 
+==== ARM system register instructions
+
+Examples of using them can be found at: <<dump-regs>>
+
+aarch64 only uses exactly 2 instructions:
+
+* MRS: reads a system register to a regular register
+* MSR: writes to the system register
+
+aarch32 is a bit more messy due to older setups, we have both:
+
+* MRS and MSR which are much like in aarch64
+* coprocessor accesses:
+** MRC: reads a system register, C means coprocessor, which is how system registers were previously known as
+** MCR: write to the system register
+** MRRC: like MRC, but used for the system registers that are marked as 64-bit, and reads to two general purpose regis
+** MCRR: write version of MCRR
+
+<<armarm8-fa>> G1.19.4 "Background to the System register interface" says that only CP14 and CP15 are specified by the ISA:
+
+____
+The interface to the System registers was originally defined as part of a generic coprocessor interface, that gave access to 15 coprocessors, CP0 - CP15. Of these, CP8 - CP15 were reserved for use by Arm, while CP0 - CP7 were available for IMPLEMENTATION DEFINED coprocessors.
+____
+
+and the actual coprocessor registers are specified at:
+
+* CP14: Table G7-1 "Mapping of (coproc ==0b1110) MCR, MRC, and MRRC instruction arguments to System registers"
+* CP15: Table G7-3 "VMSAv8-32 (coproc==0b1111) register summary, in MCR/MRC parameter order."
+
 === ARM SIMD
 
 Parent section: xref:simd-assembly[xrefstyle=full]
@@ -21909,6 +21938,11 @@ ISA quick references can be found in some places:
 
 https://static.docs.arm.com/ddi0487/db/DDI0487D_b_armv8_arm.pdf
 
+[[armarm8-fa]]
+===== ARMv8 architecture reference manual db
+
+https://static.docs.arm.com/ddi0487/fa/DDI0487F_a_armv8_arm.pdf
+
 [[armv8-programmers-guide]]
 ===== Programmer's Guide for ARMv8-A