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<title>15.1 System memory use</title>
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<h2>
<a name="1204408"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">15.1 System memory use</font>
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<a name="1204409"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The ST20-C2 has a signed address space where the address ranges from <em>MostNeg</em> (#80000000) at the bottom to <em>MostPos</em> (#7FFFFFFF) at the top. All ST20-C2 variants are guaranteed to have a small area of RAM at the bottom of the address space provided by on-chip memory. The bottom of this area is used to store various items of system state. These addresses should not be accessed directly but via the instructions that allow them to be accessed. </font>
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<p>
<a name="1204410"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Near the bottom of the address space there is a special address <em>MemStart</em>. Memory below this address is for private use by the processor while addresses above it are for use by user programs. This address varies between variants but its value can be obtained via the <strong>ldmemstartval</strong> instruction. The memory below it is used for subsystem channels and trap handlers.</font>
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<h3>
<a name="1204413"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">15.1.1 Subsystem channels</font>
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<a name="1204414"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Each channel between the processor and a subsystem is allocated a word of storage below <em>MemStart</em>. This is used by the processor to store information about the state of the channel. This information should not normally be examined directly, although debugging kernels may need to do so. Each of these channels represents a uni-directional point-to-point communication between the CPU and a subsystem.</font>
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<h3>
<a name="1204415"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">Boot channels</font>
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<a name="1204416"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Certain of the subsystem channels which are input channels are identified as `boot channels'. When the processor is reset in boot-from-link mode then the processor waits for boot commands on one of these channels.</font>
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<p>
<a name="1204417"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">To maintain compatibility with previous processors the memory map allocates the first eight words as external link channels; four input channels followed by four output channels. In addition the four input channels are used as the boot channels. A device using an ST20-C2 need not implement all four channels (indeed, if a debug control unit is used to provide boot and debug facilities, no channel need be implemented) in which case the relevant channel words are considered to be reserved.</font>
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<h3>
<a name="1204418"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">15.1.2 Trap handlers</font>
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<p>
<a name="1204419"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The area of memory reserved for trap handlers is broken down hierarchically:</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204420"> </a>each priority has a set of trap handlers,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204421"> </a>each set of trap handlers has a handler for each of the four trapgroups,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204422"> </a>each trapgroup handler has a trap routine structure and a trapped process structure,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204423"> </a>each of the structures contains four words.</font></p>
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<p>
<a name="1204426"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The format of the trap structures has been defined earlier in <a href="traps3.htm#1204553"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Section⌨️ 快捷键说明
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