I'm working with an 8051 (Cypress FX2LP) that doesn't have jtag/bdm capability. Typically, developers on this project have been using ad-hoc serial printfs for debugging. I'm looking into options for serial debug monitors such as Keil's Mon51, Isd51 or IAR's generic ROM-monitor.

I'll need to modify/configure this debug monitor to write to code RAM (to set software breakpoints). I'd guess that most 8051 debug monitors offer the ability for such modifications in order to support Harvard architecture or bank switching.

The Keil tools come with Mon51 and Isd51. The Isd51 is very small and comes as source that can optionally be embedded in the application. This can all be done with the free Keil eval (subject to the eval size limitations).

In the datasheet of MSC1210(http://www.kynix.com/uploadfiles/pdf2286/MSC1210Y2PAGR.pdf), it explains some strategies for serial debugging on an 8051.

If this Cypress part had jtag/bdm - it would be a no-brainer - I'd use that.

Does anybody have recommendations for serial debuggers for 8051 or similar processors?

Have you had to modify it to write to Harvard code RAM or flash etc?