From Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 36:
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Reverence toward the blessed sacrament
- No one who enters a church should fail to adore the blessed sacrament, either by visiting the blessed sacrament chapel or at least by genuflecting.
Similarly, those who pass before the blessed sacrament genuflect, except when they are walking in procession."
The 2002 Roman Missal changed this slightly. From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
ā
Genuflections and Bows
274. A genuflection, made by bending the right knee to the ground, signifies adoration, and therefore it is reserved for the Most Blessed Sacrament, as well as for the Holy Cross from the solemn adoration during the liturgical celebration on Good Friday until the beginning of the Easter Vigil.
During Mass, three genuflections are made by the priest celebrant: namely, after the showing of the host, after the showing of the chalice, and before Communion. Certain specific features to be observed in a concelebrated Mass are noted in their proper place (cf. above, nos. 210-251).
If, however, the tabernacle with the Most Blessed Sacrament is present in the sanctuary, the priest, the deacon, and the other ministers genuflect when they approach the altar and when they depart from it, but not during the celebration of Mass itself.
Otherwise all who pass before the Most Blessed Sacrament genuflect, unless they are moving in procession.
Ministers carrying the processional cross or candles bow their heads instead of genuflecting.ā