docs: Document simulation using VCS

docs/tg/sim.md

@@ -5,6 +5,7 @@ This page describes how to simulate Cheshire to *execute baremetal programs*. Pl
 We currently provide working setups for:
 
 - Questa Advanced Simulator (QuestaSim) `>= 2022.3`
+- VCS `>= 2024.09`
 
 We plan on supporting more simulators in the future. If your situation requires it, simulating Cheshire on other setups should be straightforward.
 
@@ -21,21 +22,21 @@ We provide a SystemVerilog testbench for `cheshire_soc` running baremetal progra
 
 Preloading boot modes expect an ELF executable to be passed through `BINARY`, while autonomous boot modes expect a disk image (GPT formatted or raw code) to be passed through `IMAGE`. For more information on how to build software for Cheshire and its boot process, see [Software Stack](../um/sw.md).
 
-The `SELCFG` environment variable selects the Cheshire configuration used in simulations. Possible configurations are specified in the `tb_cheshire_pkg` package. If not set or set to `0`, the default configuration is selected.
+The `SELCFG` variable selects the Cheshire configuration used in simulations. Possible configurations are specified in the `tb_cheshire_pkg` package. If not set or set to `0`, the default configuration is selected.
 
 | `SELCFG` | Configuration in (`tb_cheshire_pkg`)      |
 | -------- | ----------------------------------------- |
 | 0        | Default configuration from `cheshire_pkg` |
 | 1        | AXI-RT-enabled configuration              |
 | 2        | CLIC-enabled configuration                |
 
-The `USE_DRAMSYS` environment variable controls whether simulations are linked against and use DRAMSys for DRAM simulation. Note that before starting a simulation using DRAMSys, it must be built with `make chs-dramsys-all` first.
+The `USE_DRAMSYS` variable controls whether simulations are linked against and use DRAMSys for DRAM simulation. Note that before starting a simulation using DRAMSys, it must be built with `make chs-dramsys-all` first.
 
 For simulation of Cheshire in other designs, we provide the module `cheshire_vip` encapsulating all verification IPs and their interfaces. For details, see [Verifying Cheshire In-System](integr.md#verifying-cheshire-in-system).
 
 ## QuestaSim
 
-After building Cheshire, start QuestaSim in `target/sim/vsim` and run:
+Variables are read from QuestaSim's Tcl environment. After building Cheshire, start QuestaSim in `target/sim/vsim` and run:
 
 ```tcl
 # Preload `helloworld.spm.elf` through serial link
@@ -52,3 +53,22 @@ run -all
 ```
 
 The design needs to be recompiled only when hardware is changed. The simulation can be restarted by re-sourcing `start.cheshire_soc.tcl`, allowing binary (or image) and load method changes beforehand.
+
+## VCS
+
+Variables are read from your shell environment. After building Cheshire, start a POSIX-compliant shell in `target/sim/vcs` and run:
+
+```sh
+# Preload `helloworld.spm.elf` through serial link
+export BINARY="../../../sw/tests/helloworld.spm.elf"
+export BOOTMODE=0
+export PRELMODE=1
+
+# Compile design
+./compile.cheshire_soc.sh
+
+# Start and run simulation
+./start.cheshire_soc.sh
+```
+
+The design needs to be recompiled only when hardware is changed. The simulation can be run repeatedly using `start.cheshire_soc.sh`, allowing binary (or image) and load method changes beforehand.