Did anyone try this for GCC?

Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.

Oh realllly? Can you tell us any more about that, such as any instructions you've figured out thus far that let it be poked, or routines that use it that could be used to recover instruction opcodes?

There are Prizm BCD syscalls in Simon's mini-SDK documentation that should reveal more.

This is some (outdated) info for legacy devices:
customization of the legacy systems
The legacy systems are 7705-based. The identification of the customized processors is 7337(G) and 7355(GII), resp.

The BCD-arithmetic-registers are not documented in the 7705 hardware manual.
Perhaps a customized processor enhancement (7337; with OS 2.00 the ID has been changed to 7355) or a separate piece of hardware.
The BCD-unit is able to add or subtract two 8-digit (32 bits) BCD-numbers.

A44C0014, 32bit, BCD operand 1
A44C0018, 32bit, BCD operand 2
A44C0010, 16bit, command and status
A44C001C, 32bit, BCD result

write a value to operand 1
write a value to operand 2
write a command
read the result

typical addition sequence of two 24 digit BCD values at 0x80013E00, OS 1.03.
typical subtraction sequence of two 24 digit BCD values at 0x80013EEE, OS 1.03.

the following command-bits are known:
bit 0: 1=add, 0=subtract
bit 1: 1=use carry of the previous operation, 0=do not
bit 2: 1=use carry in any case, 0=do not

the following status-bits are known:
bit 12: carry of the previous operation
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