53 lines
3.3 KiB
Plaintext
53 lines
3.3 KiB
Plaintext
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<<<
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:sectnums:
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==== Instruction Memory (IMEM)
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[cols="<3,<3,<4"]
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[frame="topbot",grid="none"]
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|=======================
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| Hardware source file(s): | neorv32_imem.entity.vhd | entity-only definition
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| | mem/neorv32_imem.default.vhd | default _platform-agnostic_ memory architecture
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| | mem/neorv32_imem.legacy.vhd | alternative legacy-style memory architecture
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| Software driver file(s): | none | _implicitly used_
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| Top entity port: | none |
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| Configuration generics: | `MEM_INT_IMEM_EN` | implement processor-internal IMEM when `true`
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| | `MEM_INT_IMEM_SIZE` | IMEM size in bytes (use a power of 2)
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| | `INT_BOOTLOADER_EN` | use internal bootloader when `true` (implements IMEM as _uninitialized_ RAM, otherwise the IMEM is implemented an _pre-intialized_ ROM)
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| CPU interrupts: | none |
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|=======================
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Implementation of the processor-internal instruction memory is enabled by the processor's
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`MEM_INT_IMEM_EN` generic. The total memory size in bytes is defined via the `MEM_INT_IMEM_SIZE` generic.
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Note that this size should be a power of two to optimize physical implementation. If the IMEM is implemented,
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it is mapped to base address `0x00000000` by default (see section <<_address_space>>).
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By default the IMEM is implemented as true RAM so the content can be modified during run time. This is
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required when using the bootloader (or the on-chip debugger) so it can update the content of the IMEM at
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any time. If this feature is not required the IMEM can be implemented as _pre-intialized_ ROM so that the
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application code permanently resides in memory. This is automatically implemented when the
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processor-internal bootloader is disabled (`INT_BOOTLOADER_EN` = `false`).
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When the IMEM is implemented as ROM, it will be initialized during synthesis with the actual application program
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image. The compiler toolchain provides an option to generate and override the default VHDL initialization file
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`rtl/core/neorv32_application_image.vhd`, which is automatically inserted into the IMEM. If the IMEM is implemented
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as RAM (default), the memory block will **not be initialized at all**.
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.Memory Size
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[IMPORTANT]
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If the configured memory size (via the `MEM_INT_IMEM_SIZE` generic) is **not** a power of two the actual memory
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size will be auto-adjusted to the next power of two (e.g. configuring a memory size of 60kB will result in a
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physical memory size of 64kB).
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.VHDL Source File
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[NOTE]
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The actual IMEM is split into two design files: a plain entity definition (`neorv32_imem.entity.vhd`) and the actual
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architecture definition `mem/neorv32_imem.default.vhd`. This **default architecture** provides a _generic_ and
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_platform independent_ memory design that infers embedded memory blocks (blockRAM). The default architecture can
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be replaced by platform-specific modules in order to use platform-specific features or to improve technology mapping
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and/or timing. A "legacy-style" memory architecture is provided in `rtl/mem` that can be used if the synthesis does
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not correctly infer blockRAMs.
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.Read-Only Access
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[NOTE]
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If the IMEM is implemented as true ROM any write attempt to it will raise a _store access fault_ exception.
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